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Policy Analysis

Unconventional Oil and Gas Spills: Risks, Mitigation Priorities, and
State Reporting Requirements
Lauren A. Patterson,*,† Katherine E. Konschnik,‡ Hannah Wiseman,§ Joseph Fargione,∥
Kelly O. Maloney,⊥ Joseph Kiesecker,# Jean-Philippe Nicot,∇ Sharon Baruch-Mordo,# Sally Entrekin,○
Anne Trainor,◆ and James E. Saiers¶

Nicholas Institute for Environmental Policy Solutions, Duke University, 2111 Campus Drive, Durham North Carolina 27708, United

Environmental Policy Initiative, Harvard Law School, #4123 Wasserstein Hall, Cambridge, Massachusetts 02138, United States
Florida State University College of Law, 424 W. Jefferson Street, Tallahassee, Florida 32306, United States

The Nature Conservancy, 1101 West River Parkway, Suite 200, Minneapolis, Minnesota 55415, United States

U.S. Geological Survey, Leetown Science Center, Kearnevsville, West Virginia 25430, United States
The Nature Conservancy, Global Lands Team, 117 E. Mountain Avenue, Suite 201, Fort Collins, Colorado 80524, United States

Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Building 130,
Austin, Texas 78758, United States

Department of Biology, University of Central Arkansas, 201 Donaghey Avenue, Conway, Arkansas 72035, United States

The Nature Conservancy, African Program, University of Cincinnati, Department of Biological Sciences, 820G Rieveschl Hall,
Cincinnati, Ohio 45221, United States

School of Forestry and Environmental Studies, Yale University, 195 Prospect St., New Haven, Connecticut 06511, United States
S Supporting Information

ABSTRACT: Rapid growth in unconventional oil and gas (UOG)
has produced jobs, revenue, and energy, but also concerns over spills
and environmental risks. We assessed spill data from 2005 to 2014 at
31 481 UOG wells in Colorado, New Mexico, North Dakota, and
Pennsylvania. We found 2−16% of wells reported a spill each year.
Median spill volumes ranged from 0.5 m3 in Pennsylvania to 4.9 m3 in
New Mexico; the largest spills exceeded 100 m3. Seventy-five to 94%
of spills occurred within the first three years of well life when wells
were drilled, completed, and had their largest production volumes.
Across all four states, 50% of spills were related to storage and moving
fluids via flowlines. Reporting rates varied by state, affecting spill rates
and requiring extensive time and effort getting data into a usable
format. Enhanced and standardized regulatory requirements for
reporting spills could improve the accuracy and speed of analyses to identify and prevent spill risks and mitigate potential
environmental damage. Transparency for data sharing and analysis will be increasingly important as UOG development expands.
We designed an interactive spills data visualization tool (http://snappartnership.net/groups/hydraulic-fracturing/webapp/spills.
html) to illustrate the value of having standardized, public data.

energy development.3,4 Notwithstanding the incredible potential for energy development, UOG extraction activities have
raised concerns about their potential environmental impacts,
including dewatering streams as well as surface and groundwater pollution.5−7 Determination of the ecological impacts of
UOG development is considered a top science priority to


Global demand for energy, directives to reduce carbon dioxide
emissions, and technological advancements in horizontal
drilling and hydraulic fracturing, have spurred a rapid increase
in alternative and unconventional energy production over the
past decade.1 Despite a recent slowdown of activity, energy
development will likely continue; with shale gas and tight oil
play production estimated to grow 60% by 2040 compared to
2015 production.2 Unconventional oil and gas (UOG)
produced from reservoirs with low porosity and permeability,
particularly shale reserves, will be a key component of future
© XXXX American Chemical Society

Received: November 14, 2016
Revised: February 3, 2017
Accepted: February 13, 2017


DOI: 10.1021/acs.est.6b05749
Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Policy Analysis

Environmental Science & Technology

Figure 1. Location of UOG wells and reported spills.

affect which spills are reported and what information are
Spill data collected by states provide a rich opportunity for
gaining insight into when spills are most likely to occur, where
they are most likely to occur, and the underlying causes. Better
insight into these three factors would provide regulatory
agencies and industry decision-makers with important
information on where to target efforts for locating and
preventing future spills. However, due to different reporting
requirements, the types of spills reported and details about the
type, volume, location, timing and cause of the spills reported
vary across states. Moreover, some states collect or store data in
hard copy or image pdfs, inhibiting analysis unless personnel
later digitize the data. As UOG expands, efforts to reduce spill
risk would benefit from making data more uniform, accessible,
and informative to well operators, regulators, and the public.
Objectives. In this paper we cleaned and analyzed state spill
databases related to unconventional oil and gas (UOG) wells in
four states: Colorado,14 New Mexico,15 North Dakota,16 and
Pennsylvania17 (Figure 1). We defined UOG wells as those
having been hydraulically fractured, as explicitly identified in
the databases we acquired, or, where this information was
unavailable, as indicated by horizontal drilling or water use.
These states were selected because of their significance as oil or
natural gas producing states and because extensive spill data
could be obtained. We assessed data availability for 11 other
states; however the data were either difficult to obtain or we
could not determine which wells were linked to UOG activity,
leaving four states in this study. We focused on UOG wells as a
manageable subset of oil and gas related spills and one of
growing importance as UOG activity increases. Although some
of the spills that occur at UOG wells are unique to UOG

inform energy policy and conservation and management of
natural systems.8
Hydraulic fracturing enables resource extraction from both
conventional and UOG wells by injecting high-pressure fluids
mixed with chemical additives to fracture nonporous, lowpermeability rock and release the trapped gas or oil.9,10 One of
the public’s greatest concerns is the impact of surface spills−
particularly of the chemicals used in hydraulic fracturing−to the
Federal law establishes common minimum reporting requirements for some but not all spills (Supporting Information (SI),
Section A). The Clean Water Act and Oil Pollution Act require
reporting of discharges that create a sheen on navigable waters.
The Comprehensive Environmental Response, Compensation,
and Liability Act (CERCLA) and the Resource Conservation
Recovery Act (RCRA) require reporting of spills of hazardous
substances above a threshold volume, but exempt fracturing
fluids and flowback from this requirement. CERCLA also
excludes oil spills from reporting requirements. Beyond these
minimum federal standards, in most cases states determine
when and how oil and gas related spills are reported
(exceptions are for wells on federal or Tribal lands, or on the
Outer Continental Shelf). States often use “spill” and “release”
interchangeably; hereafter, we generically use “spill”.12,13 State
law specifies the type of spill event that triggers a reporting
requirement usually based on whether the spill exceeded a
threshold volume or concentration and whether it was
contained or migrated beyond the well site. These rules also
indicate the reporting method (i.e., verbal or written), timing,
and content. These factors, as well as the frequency of
inspection by regulators for independent detection of spills,

DOI: 10.1021/acs.est.6b05749
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given that only a subset of all spills are required to be reported
to each state.
Colorado. In 2000, an interagency memorandum required
companies to verbally report any spill entering or threatening to
impact surface waters as soon as practicable.24 In addition,
companies were required to report spills exceeding 5 barrels
(210 gal) on state Form 19 within 10 days of discovery (SI
Figure S2). In 2014, the reporting thresholds were lowered and
time periods for reporting were shortened;25 requiring any spill
or release greater than 1 barrel (42 gal) that escapes secondary
containment to be reported. Also in 2014, Colorado amended
Form 19 to capture information about the material spilled and
the cause for the spill.25 Spills data were obtained through the
Oil & Gas Conservation Commission.
New Mexico. New Mexico’s Oil and Conservation Division’s
2001 rules required companies to verbally report within 24 h
any “major release” of oil, gas, produced water, condensate, oil
field waste including regulated radioactive materials or other oil
field related chemicals, contaminants, or mixtures.26 A major
release was defined as greater than 25 barrels (1050 gal), or a
release that resulted in a fire or in substantial damage to
property or the environment, would reach a water course, or
might “with reasonable probability endanger public health” or
water quality.27 In addition to the verbal reports for major
releases, written reports were required within 15 days on Form
C-141 to document any “major release” or “minor release”, that
is, releases of at least 5 barrels (210 gal). The location and cause
of the spill must be reported, along with the material and the
source of the release, as well as whether the release reached a
waterway. The Division reorganized the rules in 2008, but the
substantive reporting provisions remained the same.27 Spills
data were obtained from the Oil Conservation Division.
North Dakota. Prior to 2010, companies in North Dakota
had to report “any fire, leak, spill, blowout, or release of fluid”
except for spills less than 1 barrel (42 gal) that stayed on site.28
Reports, which were strictly verbal, had to include the location
and cause of the incident, and the amount and type of fluid
involved. Effective April 1, 2010, a time limit was placed on this
verbal report (within 24 h of discovery); in addition, a written
report was required to be submitted within 10 days of cleanup
“unless deemed unnecessary by the director” of oil and gas of
the Industrial Commission.29 Effective April 1, 2014, North
Dakota added an online reporting requirement within 24 h of
discovery.30 Spills data were obtained from the Oilfield
Environmental Incident reports provided by the Department
of Health-Environmental Health. These data do not include
RCRA spills.
Pennsylvania. Pennsylvania’s 2001 rules required companies
to report by telephone to the Department of Environmental
Protection any “reportable release of brine” or the discharge of
any substance which would endanger downstream users of
water, result in or create a danger of pollution of Pennsylvania
waters, or damage property.12,31,32 The report had to include
the location and cause of the incident. “Reportable release of
brine” was defined as “spilling, leaking, emitting, discharging,
escaping or disposing” of at least 5 gallons in 24 h of brine
containing more than 10,000 mg/L total dissolved solids
(TDS), or of at least 15 gallons of brine with a lower TDS
concentration. In October 2016, Pennsylvania’s new rules went
into effect; these will require written spill reports. 33
Pennsylvania does not have a separate spill data set, therefore
spill data for our analysis were pulled from the Department of
Environmental Protection’s notice of violations (NOV) data-

development, such as spills of fracturing chemicals and
flowback, other spills, such as spills of produced water, can
occur at both conventional and unconventional wells.
Our objective was two-fold. First, we assessed the regulatory
framework for how spills were reported in each state and how
that shaped the data collected. Second, we quantified (1) the
probability of a spill by well age (with spud−initial drilling−
year representing age 0), (2) the risk of a spill occurring at
different locations on a well pad, (3) the volume released, and
(4) the underlying cause of the spill. The regulatory assessment
and the spill data analysis were used to ascertain how improved
data collection and standardization procedures might enable
higher resolution data analysis that could decrease the
likelihood for spills. We built an interactive database to
showcase this opportunity (http://snappartnership.net/

We obtained state oil and gas spills data for spills occurring
between 2005 and 2014 at UOG wells that were spudded as
early as 1995. We constrained UOG well spud dates to limit the
range of technology and techniques to unconventional shale
well activity18 and capture the transition to mainstream UOG
activity. Analysis of spills data was limited by difficulties
obtaining earlier data and little UOG production related to
shale gas or tight oil plays occurred prior to 2005.2 Spill data
were matched by American Petroleum Institute (API) numbers
to UOG wells. Data for Colorado UOG wells were obtained
from the IHS Enerdeq database,19 and from state databases in
New Mexico,20 North Dakota,21 and Pennsylvania.22 When the
distinctions between conventional and UOG wells were not
clear, we did not include the well to avoid including non-UOG
wells in our data set. Pennsylvania was the only state to provide
a separate well database for unconventional wells. The IHS data
for Colorado provided the volume used to complete wells so
we included all horizontal wells and those wells using more
than 1 Mgal for completion. In North Dakota and New Mexico,
only horizontal wells were included. Further detail on how we
identified UOG wells in each state can be found in Section B of
the Supporting Information.
Spill Data. Spills are events in which an unauthorized
discharge released material, regardless of whether the spill
stayed within the boundaries of the pad or migrated into
groundwater or surface waterways. For a spill to occur there
must be (1) a means by which materials were released from a
particular point at or near the well site (hereafter referred to as
a pathway) and (2) a causal mechanism (such as human error
or equipment failure) that resulted in a release from the
particular pathway. For example, a tank (pathway) may release
produced water to the surface due to equipment failure (causal
mechanism) from a valve malfunction. We focused on
identifying the pathway and the causal mechanism for each
spill to determine the most likely routes for spills and thereby
suggest where monitoring, development of new practices, and
focused interventions may have the most benefit. A detailed
analysis of the materials spilled and potential environmental
impacts of spills is described in a separate paper.23
The best source of information regarding UOG spills are
state reporting records. Each state’s reporting thresholds and
requirements differ, such that the subset of spills that are
reported vary by state (SI, Section B provides details on state
spills data). The number of spills reported is likely conservative

DOI: 10.1021/acs.est.6b05749
Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 2. Common pathways for spills.

wellhead, and unknown (Figure 2; SI Table S3). Across all
states, between 93% (PA) and 98% (ND) of spills fell into
these main pathways.
Of the four states assessed in this study, only Colorado and
New Mexico categorize the underlying cause of a spill in their
aggregated data sets. We folded their standardized designations
into four general categories: equipment failure, environmental
conditions, human error, and unknown. We used narratives and
other reported data from North Dakota and Pennsylvania to
assign a causal category to each spill in those states as well.
Equipment failures ranged from leaks due to corrosion to
specific mechanical problems within equipment such as valves,
flanges, gaskets, sight glasses, fire tubes, and polish rods. Spills
from equipment that failed due to extreme conditions, such as
cracked valves due to freezing, were listed as spills caused by
environmental conditions. Other environmental conditions
included flooding, heavy rain, snowmelt, high winds, rock
slides, well kicks (incidents in which pressure causes brines and
other fluids to enter the wellbore), valves opened by cattle
coming into contact with equipment, and gophers chewing
through flowlines. Human errors included incorrect valve
positions, miscommunication between well operators and
transportation staff, flowlines being punctured by construction
equipment, vandalism, and illegal dumping.
Analysis. We normalized the data four different ways to
compare spill rates between states: the average rate from 2005
to 2014, annual rate, life-year rate, and the combination of
annual and life-year rates. We use the term “rate” to refer to the
occurrence rate (frequency) of spill events. Examples for all
calculations can be found in the SI Calculations spreadsheet.
We calculated the average spill rate by dividing the total
number of spills by the number of well-years observed within
the state. A well-year is observed each year since the well was
drilled and spill data were obtained (2005−2014). For example,
a well drilled in 2010 would have five well-years observed (from
2010 to 2014) for a spill in 2014, but if that well had been
drilled prior to 2005, it would have 10 well-years observed
(from 2005 to 2014).

base for UOG (SI, Section B). This necessarily limited the spill
data to those where an inspector issued an NOV, possibly
leading to an underestimation of the number spills in our
analysis. Using the violation code and comments, we included
in our analysis only those NOV’s categorized as having a spill or
the potential to result in a spill (n = 1293).
Data Cleaning. To ensure consistency among states, the
same analyst cleaned each state database. The data provided by
each state were reconciled with a consistent classification
system for the volume units, material, pathway, and cause of
spill. Some states provided the data in separate fields, while
others only provided the information through narrative
descriptions. When there were conflicting data for a spill
(e.g., the cause or volume of the spill differed from the
narrative), the narrative was taken to be correct. Spill volumes
were converted from reported units (typically barrels) into
cubic meters and gallons. The material spilled was categorized
as drilling waste, chemicals, hydraulic fracture solution,
saltwater, freshwater, oil products, diesel, equipment oil, and
unknown. We employed similar methods to those reported in
an EPA analysis of eight states34 in that we used one analyst
and treated narrative descriptions as the governing factor to
identify common pathways and causal mechanisms. However,
we focused on spills at all stages of well development while the
EPA analysis focused on spills connected to the hydraulic
fracturing process.
Pathway categories were developed based on descriptive
narratives of incidents. Pathways identify the specific point on
or near the well site at which a spill occurred, except the
blowout pathway, which describes an event in which pressure
builds up in the wellbore and can lead to underground or
surface releases. Pathways that were relatively small and
occurred infrequently were lumped into a category of “other”
(SI, Section C). More commonly reported pathways include:
blowouts, drilling equipment (active mud system, drill rig, and
shakers), completion equipment (blender, chemical totes, and
storage containers), tanks, pits, flowlines, heater treaters,
stuffing boxes, pumps, transportation, leaks around the

DOI: 10.1021/acs.est.6b05749
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We calculated the annual spill rate by dividing the number of
spills in a given year by the cumulative number of wells drilled,
including that year (see SI Calculations spreadsheet).
Given the rapid and recent expansion of unconventional
wells, most of our observations are for young wells (median
well age in 2014 was 3 years). It may not be appropriate to
extrapolate this spill rate over the entire lifetime of a well.
Therefore, we also calculated the life-year spill rate for each lifeyear of a well (i.e., all new wells, all one year old wells, etc.). We
based the life-year of a well on its spud date. We rounded spills
to the closest life-year, such that life-year 1 represents spills that
occurred between six and 18 months after a spud date. Lifeyears ranged from 0 (wells spudded in 2014) to 19 (wells
spudded in 1995) (SI Table S7). While spills may have
occurred at some of these wells prior to 2005, we only have
spill data from 2005 to 2014.
The volume of material spilled was assessed using quantile
regressions (quantreg package in R) at 10% intervals. For each
state, a chi-square test was performed between the observed
and expected number of wells with multiple spills. The
expected number of wells is based on the binomial probability
of more than one spill occurring at a well.

Figure 3. Annual spill rate. Dashed vertical lines represent changes in
reporting requirements.

analysis, and we did not discern a noticeable change in spill
rates from those few months of data. The requirement that the
spill escape secondary containment may also drive down spill
reports. Colorado also provided unique identifying information
for pits, tanks, flowlines, etc., but it did not always link these
spills to an individual well. This reporting method may have
contributed to lower Colorado spill rates since we only
included spills that could be linked to an unconventional well
Pennsylvania’s reporting requirements remained the same
throughout this time period; however, a spike in UOG well
development in 2009 corresponded with a doubling of the
number of inspectors from 35 to7635 and a spike in annual spill
rates (Figure 3). The rates declined as the number of wells
increased (1208 wells in 2009 to 4478 wells in 2011), due to a
combination of factors including the centralization of regulators
into a new Office of Oil and Gas Management within the
Pennsylvania Department of Environmental Protection, operators gaining more experience with best management practices,
and a shift in inspection to focus on drilling which increased the
number of wells that could be inspected while simultaneously
decreased the likelihood of finding a violation.36 Frequency of
inspection is a critical factor for reporting spills in Pennsylvania
since spills data were obtained from notices of violations.
Meanwhile, pit spills decreased from 132 in 2010 to 28 in 2011,
which may correspond with an increased effort by the
Pennsylvania Department of Environmental Protection to
enforce pit regulations (S. Perry, Pennsylvania Department of
Environmental Protection, Personal Communication, May 25,
2016). Spill rates increased in New Mexico between 2012 and
2013, corresponding with increased production. Over this
period, storage was inadequate to contain the increased
quantities of produced water and oil, and some truckers
determined it was cheaper to illegally dump produced water
than to find adequate storage (D. Sanches, New Mexico Oil
Conservation Division, Personal Communication, June 1,
2016), which may have accounted for the increased spill rate.
Life-Year Spill Rates. The spill rate by life-year of the well
was greatest in the first few years (SI Table S8) when drilling,
completion, and the highest amount of production occurred.37,38 The average spill rate ranged between 2%
(Colorado) to 15% (North Dakota) during the first three
years of well life; with rates decreasing as the well matured. We
quantified the spill rate by life-year when at least 100 well years
were present to increase the robustness of the analysis (Figure
4). Our data set only included new wells spudded from 1995;
further research is needed on spill rates for older wells
exceeding 20 years of age.

Average Spill Rates. Between 2005 and 2014 there were
6648 spills reported across the four states based on each state’s
reporting requirements and our definition of UOG wells (SI
Table S4). Our results exceed the number of spills found by
EPA (n = 457) for eight states between 2006 and 2012 because
we included spills that occurred during all stages of unconventional production (from drilling through production) while
EPA focused on those spills explicitly related to hydraulic
fracturing.34 In our study, North Dakota reported the most
spills (n = 4453) and the highest overall spill rate at 12.2% (SI
Table S6). Pennsylvania reported 1293 spills (4.3%), whereas
New Mexico (426 spills; 3.1%) and Colorado (476 spills; 1.1%)
each reported fewer than 500 spills. Some wells within each
state experienced multiple spills (from 0.12% of wells in
Colorado to 7.32% of wells in North Dakota); and these wells
with multiple spills contributed to a large proportion of spills.
The proportion of spills coming from a well with more than
one spill accounted for 26% of all spills in Colorado, 37% in
New Mexico, 53% in North Dakota, and 47% in Pennsylvania.
The number of wells with multiple spills was 1.3 (North
Dakota) to 4.6 (Colorado) times higher than expected due to
chance alone, indicating that wells with one spill have a higher
probability of spilling in the future (SI Table S5).
Annual Spill Rates. Within New Mexico and Colorado the
annual spill rate fluctuated within a percent or two; whereas in
North Dakota and Pennsylvania, the annual spill rate fluctuated
by more than 13% (SI Table S6). Fluctuations were likely
influenced by changes to state spill reporting requirements,
demonstrating how state policies directly impact efforts to
identify and accurately assess UOG risk, their causes and
potential mitigating remedies. For instance, North Dakota’s
transition from a verbal notification to a written reporting
requirement coincides with an increase in spill rates of 3−4%
after 2010 (Figure 3). Similarly, the lowering of a reporting
threshold for a spill could increase the number of reported
spills. From 2000, Colorado only required spills larger than 210
gallons to be reported; after 2014, the state lowered the
threshold to 42 gallons when a spill escaped secondary
containment. This change occurred during our last year of

DOI: 10.1021/acs.est.6b05749
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Figure 4. Spill rates by life-year of well (left) and number of UOG wells that experienced each life-year (right). New wells included those with a life
year of 0−2 years (3 total years).

Figure 5. Spill rate by life-year of the well. Bubble size is scaled to the maximum spill rate that occurred in each state. The maximum spill rate is given
(4% in CO, 9% in NM, 23% in ND, and 12% in PA).

Annual Spill Rate by Life-Year. Given the observed
change in spill rate between younger and older wells, it was
necessary to control for life-year to understand changes in spill
frequency during the decade we analyzed. We examined spill
rate by year and life-year to differentiate whether spill rates
were influenced by improvements in technology or management practices over time or by the age of the well. For
Colorado, North Dakota, and Pennsylvania the maximum spill
rate was consistently within the first three life-years of the well
over time (Figure 5). New Mexico experienced an increase in
spill rate across all life-years in 2013 when production began to
rapidly increase.

Spill Volume. Reported volumes of spills ranged from as
small as 0.004 m3 (1 gal) to as large as 3756 m3 (991 200 gal).
Spill volume was not always reported. The frequency of a spill
report including the volume spilled varied between states,
ranging from 27% in Pennsylvania to 98% in North Dakota (SI
Table S9). Pennsylvania may have “missing” volumes data
because reporting of spills has only been required by telephone;
agency guidance discouraged written notification.39 The 2016
regulations will require a written report for spills exceeding 42
gallons or when a spill threatens to pollute Pennsylvania
waters.33,40 Another reason for missing volumes data is that the
spills data for Pennsylvania comes from its enforcement

DOI: 10.1021/acs.est.6b05749
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records. Inspectors may not have known the volume of spills
encountered after the fact, or may have observed a potential
spill that had not yet manifested into an actual spill.
When volumes were reported, the median volume of
reported spills in Colorado (3 m3 or 798 gal) and New Mexico
(4.9 m3 or 1302 gal) were larger than North Dakota (0.8 m3 or
210 gal) and Pennsylvania (0.5 m3 or 120 gal) (Figure 6). The
larger median sizes in Colorado and New Mexico may reflect
the higher volume reporting threshold relative to Pennsylvania
and North Dakota (SI Figure S2).

Figure 7. Number of spills reported for each state by pathway
category. Spills reported as pits or tanks in Pennsylvania were divided
equally between the two categories.

(N = 519 spills), with the majority of those spills related to
issues of loading and unloading material (N = 452; 87%).
North Dakota experienced a large number of spills related to
heater treaters and stuffing boxes (N = 682), while
Pennsylvania had a high number of pit-related spills (N =
345), the majority of which occurred prior to 2011.
The data indicate that for every 1000 wells drilled, 12 wells
experienced a tank related spill and 11 wells experienced a spill
related to flowlines (Table 1). Approximately one well per
thousand had a blowout, which is comparable with reported
unconventional blowout frequencies of 1−3 per 1000 wells in
Texas42 and 1 blowout per 1000 onshore wells drilled between
1975 and 1990.43 In another study, the number of blowouts
reported at California oil and gas wells decreased by 80% to 0.2
blowouts per 1000 wells due to improvements in production
practice.44 Issues related to well casing and well communication
are rare (<1 per 1000 wells) according to our data. However,
underground releases are likely under-represented in the spills
database because it is difficult to ascertain when a subsurface
spill occurred. For example, the Notice of Violations database
in Pennsylvania found that between 2.6% and 6.2%10 of wells
had structural integrity issues, which was much higher than
reported in the spill data of the other three states (Table 1). In
North Dakota and Pennsylvania, the pathway was unknown in
24 and 16 of 1000 spills, respectively. In Pennsylvania,
unknown was the predominant pathway.
Reported spill volumes varied widely for most pathways (SI
Figure S4). The largest spill reported in all four states was a
freshwater tank spill of 3752 m3 (991 200 gal) in North Dakota.
North Dakota also received reports of a saltwater flowline leak
and another freshwater tank leak that each released more than
2688 m3 (710 000 gal). The maximum reported spill in
Colorado was attributed to a blowout with 731 m3 (193 200
gal) released followed by a pit spill of 636 m3 (168 000 gal).
New Mexico’s largest spills were related to the wellhead; for
instance, a spill of 372 m3 (98 280 gal) resulted from well
communication, while a blowout released 364 m3 (96 600 gal).
Pennsylvania had by far the smallest number of spill volumes
reported, making it unclear whether the relatively low
maximum spill volume of 67 m3 (17 640 gal) from a tank is

Figure 6. Quantiles of reported spill volumes by state. For instance,
half of spill volumes in New Mexico were less than 4.9 m3, while half
were less than 0.5 m3 in Pennsylvania.

The total volume of reported spills from 2005 to 2014 was
970 m3 (0.26 Mgal) in Pennsylvania, 5373 m3 (1.42 Mgal) in
Colorado, 6562 m3 (1.73 Mgal) in New Mexico, and 33 850 m3
(8.94 Mgal) in North Dakota. If we took the median spill
volume by state and applied it to those spills without a reported
volume, these estimates increase to 1447 m3 in Pennsylvania,
5488 m3 in Colorado, 6621 m3 in New Mexico, and 33 937 m3
in North Dakota. There were 46 reported freshwater spills, of
which 41 occurred in North Dakota, including 10 spills that
exceeded 100 m3 (26 417 gallons). The reported volume spilled
in PA would fill 40% of an Olympic-sized pool; removing
freshwater spills, the volume reported in North Dakota would
fill 8.8 Olympic-sized pools. If we limit our analysis to those
materials associated with hydraulic fracturing (chemicals,
hydraulic fracture solution, and flowback), the total volume of
reported spills ranged from 377.2 m 3 (0.1 Mgal) in
Pennsylvania to 769.3 m3 (0.2 Mgal) in Colorado. A more
in-depth analysis by material and impacts to surface waters from
this data set are explored in a separate analysis.23 Unlike spill
rates, the volume of spills was not clearly related to the life-year
of the well (SI Figure S3).
Pathway Analysis. In the majority of their incident reports,
neither North Dakota nor Pennsylvania provided pathway
information, with many pathways being discerned from
narrative descriptions, resulting in 1322 unknown pathways
across the two states (Figure 7). For the spills that did have a
reported pathway, the most common pathways across all states
were related to storage (tanks = 1405 spills, pits = 322 spills,
pits or tanks =190 spills) and flowlines (N = 1393 spills);
accounting for 50% of spills. This finding comports with
research done in North Dakota that found tank and flowlines
were major pathways for spills.41 In addition, spills related to
transporting materials by trucks were prevalent in all four states

DOI: 10.1021/acs.est.6b05749
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Policy Analysis

Environmental Science & Technology
Table 1. Annual Number of Spills Per Thousand Well-Yearsa


new mexico



































heater treater
stuffing box







blowout preventer
free water knockout
well casing
well communication












north dakota

The most frequent pathway is highlighted for each state.

Causal Mechanism. In Colorado and New Mexico, the
cause of the spill is explicitly asked for in the reporting form,
resulting in nearly 90% of incident reports providing an
underlying cause. In contrast, no causal mechanism could be
attributed to the majority of spills in North Dakota and
Pennsylvania (Figure 8), where such information is not actively

determine the quantity, quality, and usability of data received by
the state.
Data Collection. For states to accurately profile the risks of
pollutant releases associated with UOG production and tailor
regulations to these risks, spill data need to be collected. States
that use low minimum volume thresholds for reporting
requirements can quantify and potentially address the
cumulative impacts of low volume spills. Standardized data
should be collected within and across states to facilitate future
analyses and guide policy, practice and mitigation. In the states
we reviewed, the reporting thresholds ranged from 5 gal
(Pennsylvania) to 210 gal (New Mexico) (SI Figure S2). A
threshold quantity for reporting is logical so that operators and
regulators are not overwhelmed by reports of de minimiz
quantities of substances being released to the environment. But
if the state spills threshold is too high, valuable information
about the types and frequencies of spills occurring will be
missed. Moreover, a number of smaller incidents at a site or,
associated with a particular company, or from conditions such
as an unusually cold winter and freezing of valves, can indicate
problems that could be addressed before a larger incident
occurs. In addition, more subjective thresholds may not be clear
enough to induce reporting. For instance, a release of less than
210 gallons in New Mexico should be reported if it “may with
reasonable probability be detrimental to water”.26 This
subjective standard is unlikely to lead to consistent reporting.
Finally, limiting reporting to spills outside of secondary
containmentas Colorado doesmay tell a more accurate
story about potential impact, but also reduces the opportunities
to identify and mitigate risks within secondary containment
before they pose a problem to the environment by escaping

Figure 8. Percent of spills by causal mechanism.

solicited. Additional insights related to volumes and rates
associated with causal mechanisms for pathways can be found
in SI (Section E).
Data Reporting and Policy Implications. The primary
regulatory responsibility for UOG development lies with the
states.45,46 This includes spill reporting requirements. Our
findings suggest that the differences in these reporting regimes

DOI: 10.1021/acs.est.6b05749
Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Policy Analysis

Environmental Science & Technology

North Dakota’s online form resulted in the type of incident
being labeled as “other”. The narratives accompanying these
spill reports may suggest additional common pathways, which
could be added to the menu.
Now that Pennsylvania’s new rules require a written report
for some spills, the Commonwealth might consider creating an
online form with standardized categories of response and, with
space for narratives to elicit details that cannot be captured
through standardized reporting.
Converting Data to Information. State spill data hold great
promise for risk identification and mitigation. About 15% of all
wells reported a spill, with more than 75% occurring within the
first three years of well life. State spill rates indicate that for
every 1000 wells there were 11 spills in Colorado to 122 spills
in North Dakota. However, this comparison is not apples-toapples given the differences in reporting thresholds between the
states, with North Dakota having a lower reporting threshold
than Colorado (SI Figure S2). Additionally, we found between
26 and 53% of spills occurred at wells that experienced more
than one spill, perhaps indicating wells with chronic problems.
The most common pathways were tanks and flowlines,
followed by pits, transportation, and heater treaters. Additional
safety practices, training and failsafe technology could
significantly reduce the prevalence of these spills;48−50 for
example, spills associated with the loading and unloading of
trucks. Based on this information, companies and agencies
might direct resources to improve supervision of the loading or
unloading process and better training of employees involved in
this process. Or, industry might focus on improving
technologies to prevent or mitigate these spills, such as well
pad liners or other secondary containment placed beneath
loading and unloading operations. Colorado recently undertook
an analysis of their spills data and found flowlines to be a major
cause of spills.51 As a result, the agency issued guidance on
flowline installation and maintenance. 52 This type of
information could also be used by the oil and gas industry to
improve internal line inspections.
Similarly, the high rate of spills from valves at the wellhead
suggests a need for better securing or protecting the valvesor
simply fencing the wellhead or well siteto prevent wildlife
and livestock from jostling the valves and causing spills. And
spill rates from flowlines cut during excavation and construction
suggest that clearer marking of flowline location and the use of
flowlines with puncture- and corrosion-resistant material could
reduce these incidents. Identification of these types of risks
could also inform leading practices for industry and permit
conditions for regulators. As UOG expands internationally,
focusing monitoring on high risk pathways will provide the best
opportunity to reduce risk from surface spills.
Further improving reporting requirements and processes for
reporting will facilitate states’ and companies’ efforts to identify
risks for certain types of spills and take action to mitigate some
of the identified risk factors.46,53 To the extent that this
information is publicly available and searchable, operators can
use it to remove or mitigate risk factors to improve
environmental performance and avoid higher insurance
Assembling these data electronically within a centralized
database would allow state regulators and other stakeholders to
identify trends, including the most common spill pathways and
causes, as well as identify the wells or operators associated with
unusually high spill rates. Making this information publicly
available and providing it in an easy, usable format would allow

Each state required reporting of the pathway of the spill;
however, only Colorado and New Mexico provide separate
fields asking operators to describe the pathway and the cause
(equipment failure, human error, etc.). Only 3% of pathways in
both states were unknown; whereas 19% of North Dakota and
36% of Pennsylvania spill pathways were unknown.
Finally, the timing for reporting is important. The sooner an
operator is asked to record a description of a spill, the more
likely the information will be accurate and based on eye-witness
accounts. Colorado, New Mexico, and North Dakota have
immediate reporting deadlines (i.e., within 24 h) for large
events, but no state requires this initial report to be in writing.
Follow-up written reports are due within 3 days in Colorado,
10 days in North Dakota, and 15 days in New Mexico. Before
October 2016, Pennsylvania’s rules did not set a deadline for
reporting; the Commonwealth’s new rules will require
reporting of larger or more harmful spills in writing within 15
days of discovery.40
Data Accessibility. Further, the data need to be accessible so
that the data can be readily analyzed to identify the most
relevant risks to the benefit of a variety of stakeholders
including industry, academia, regulators, and NGO’s. Regardless of the user, if data are not presented in a standardized,
tabular format, it is difficult to analyze spill data effectively; this
condition is prevalent across states.34,47 The biggest hurdle to
using data, as currently collected, is the time and resources
required to make it usable. Colorado, New Mexico, and North
Dakota require written reporting; as will Pennsylvania once its
October 2016 rules go into effect.33 Verbal reporting is not only
less reliable, but it places the onus on the regulator receiving
the call to take down the correct information and to upload it
into a system where it can be integrated with information from
other calls. Pennsylvania’s verbal reports were not available to
assess, requiring reliance on Pennsylvania’s notice of violation
database to identify potential spills.
Once a state requires written reporting, the next step is to
craft the reporting form to standardize the collected
information. This makes it easier to compare information
across incident reports. Online reporting eases processing and
shortens the timeline for making data available. Colorado and
North Dakota use online report submissions, with checkboxes
and drop-down menus to elicit standardized answers. This
enables a consistent taxonomy of spills to help regulators and
companies spot risk patterns more efficiently. The forms still
elicit a narrative response about the pathway of the spill and
cause of the spill, which provides greater detail and nuance than
checkboxes. The pdf forms are completed and submitted
electronically. In addition North Dakota’s form cannot be
submitted until key fields are completed, reducing the amount
of missing information. New Mexico also provides an online
form, but it must be downloaded and submitted in person or by
mail to one of four district offices where the data are then
entered electronically. Moreover, most of New Mexico’s form
calls for narrative responses to such questions as volume of the
release and a description of the incident. While narratives are
rich sources of information, much additional work by the end
user is required to make the information accessible in New
Going forward, additional information might be useful; for
instance, whether a spill occurred during construction, drilling,
fracturing and completion, production, and/or during maintenance events. States also might consider adding more options
in their drop-down menus. Approximately 14% of entries in

DOI: 10.1021/acs.est.6b05749
Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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