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Hydrology and subsurface transport of oil-field brine at the U.S. Geological Survey OSPER site “A”, Osage County, Oklahoma
Applied Geochemistry Applied Geochemistry 22 (2007) 2155–2163 www.elsevier.com/locate/apgeochem
Hydrology and subsurface transport of oil-field brine at the U.S. Geological Survey OSPER site ‘‘A’’, Osage County, Oklahoma W.N. Herkelrath
a,*
, Y.K. Kharaka a, J.J. Thordsen a, M.M. Abbott
b
a b
U.S. Geological Survey, Menlo Park, CA 94025, United States U.S. Geological Survey, Oklahoma City, OK 73116, United States Available online 5 May 2007
Abstract Spillage and improper disposal of saline produced water from oil wells has caused environmental damage at thousands of sites in the United States. In order to improve understanding of the fate and transport of contaminants at these sites, the U.S. Geological Survey carried out multidisciplinary investigations at two oil production sites near Skiatook Lake, Oklahoma. As a part of this effort, the hydrology and subsurface transport of brine at OSPER site ‘‘A’’, a tank battery and pit complex that was abandoned in 1973, was investigated. Based on data from 41 new boreholes that were cored and completed with monitoring wells, a large (200 m · 200 m · 20 m) plume of saline ground water was mapped. The main dissolved species are Na and Cl, with TDS in the plume ranging as high as 30,000 mg/L. Analysis of the high barometric efficiency of the wells indicated a confined aquifer response. Well-slug tests indicated the hydraulic conductivity is low (0.3–7.0 cm/day). Simplified flow and transport modeling supports the following conceptual model: (1) prior to the produced water releases, recharge was generally low (1 cm/a); (2) in 60 a of oil production enough saline produced water in pits leaked into the subsurface to create the plume; (3) following abandonment of the site in 1973 and filling of Skiatook Reservoir in the mid1980s, recharge and lateral flow of water through the plume returned to low values; (4) as a result, spreading of the brine plume caused by mixing with fresh ground water recharge, as well as natural attenuation, are very slow. Published by Elsevier Ltd.
1. Introduction Salt water produced as a byproduct of petroleum production can cause long-lasting environmental damage. In the early days of oil production in the United States, oil-field brine was commonly discharged into streams or brine pits (Sackett and Bowman, 1905; Kharaka and Dorsey, 2005). Although, legislation enacted in the 1970s made sur*
Corresponding author. E-mail address: [email protected] (W.N. Herkelrath).
0883-2927/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2007.04.004
face disposal of oil-field brine illegal in most states, the United States has been left with a legacy of thousands of sites that have been impacted by saline water produced from oil and gas wells. In 2001 the U.S. Geological Survey, in cooperation with the U.S. EPA, began a multidisciplinary study of the impact of oil production on the nearfield environment at two oil-field sites near Skiatook Lake in Osage County, Oklahoma. The sites have been dubbed the ‘‘Osage-Skiatook Petroleum Environmental Research’’ (OSPER) sites, ‘‘A’’ and ‘‘B.’’ OSPER site ‘‘A,’’ the focus of this investigation, is
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a legacy site (for location map see Otton et al., 2007, this issue) that started production in 1912, but has been abandoned since about 1973, although production continued elsewhere on the lease. Intensive studies of the geology, geophysics, geochemistry, hydrology and microbiology of the subsurface environment were carried out at this site (Kharaka et al., 2005; Otton et al., 2007). The goal of the investigation was to document and quantify the contaminant plumes found at the site, and to develop a better understanding of the processes that influence the fate and transport of contaminants. This paper summarizes results of hydrologic investigations and includes development of a conceptual and numerical model of subsurface brine transport at the site. 2. Site background A detailed history and description of the site geology is given by Otton et al. (2005, 2007). Geochemistry of the site was discussed by Kharaka et al. (2005). Briefly, about 120,000 barrels of oil were produced from wells near the site in the years 1913–1973. To retrieve the oil, a mixture of oil and brine from nearby wells was pumped into redwood tanks, separated, and then moved in a trench to two unlined earthen pits (Fig. 1). One pit held the oil, which was recovered by the operator. The other pit temporarily held the brine, which was discharged into an ephemeral creek downslope (north) of the pit. Otton et al. (2007) estimate that approximately 190,000 m3 of brine were produced at this site. The brine pit was roughly circular, with a diameter of approximately 15 m. A salt scar approximately 60 m wide by 100 m long north of the pit developed early on and persists in 2006. The northern extent of the scar is currently truncated by Skiatook Lake, which was filled in 1987. Geology at this site was investigated by surface mapping and coring (Otton et al., 2005, 2007). Thin sections were analyzed to estimate porosity. Relatively thin (0–1.0 m thick) surficial sediments composed of eolian sand and colluvium overlie bedrock that consists mainly of sandstone and shale. Otton et al. (2007) identified 4 stratigraphic units in the salt scar area (Units 2–5, increasing in depth from 2 to 5). Unit 2 is comprised of weathered sandstone with average porosity of about 24% and thickness of about 3 m. Unit 3, which is about 3 m thick, contains interbedded thin layers of weathered sandstone and shale. The sandstone in Unit 3 has high porosity (18%), but the shale
porosity is low (1%). Unit 4, which is about 6 m thick, is largely unweathered dolomite-cemented sandstone with variable porosity ranging from 1% to about 20%. Unit 5 is composed of interbedded muddy sandstone, siltstone, mudstone and shale. Sandstones in Unit 5 have about 12% porosity, whereas the other rocks in this unit have a median porosity of about 2%. A total of 41 boreholes were drilled, and screened wells were installed for water sampling, chemical analysis, water level monitoring, and hydraulic testing (Fig. 1). Chemical analyses were published by Kharaka et al. (2005). A Geoprobe rig was used for reconnaissance drilling and to install 25 shallow (<3 m deep), 25-mm-diameter monitoring wells (AE series, Fig. 1). Auger and rotary rigs were used to drill 16, 133-mm-diameter boreholes (AA series, Fig. 1). Two or 3 separate screened wells were installed in each auger hole, enabling multilevel water sampling and head measurements. Completions were placed in water-bearing sandstone units. Each completion consists of a 51-mm-diameter PVC pipe that runs from about 1 m above the surface down to a screened interval at the bottom of the pipe. The borehole around each screen was filled with a layer of coarse sand. Completions were hydraulically isolated from one another by filling the borehole above and below the sand layer with bentonite. Analysis of water samples from the wells indicates there is a large (200 m · 200 m · 20 m) plume of high TDS ground water that extends in the subsurface far beyond the area of the salt scar at the surface. The plume extends from well AA06 to AA10 on a north–south transect (Fig. 2), and from well AA09 to AA05 on an east–west transect (Fig. 3). TDS in the plume ranges as high as 30,000 mg/L, with Na and Cl being the main dissolved species in the contaminated plume water. Water in the plume contrasts with water samples from nearby background ground water wells, which have low salinity (150–1000 mg/L TDS) and have comparable values for the equivalent concentrations of Na, Mg and Ca, as well as for Cl, SO4 and HCO3. Produced water samples from nearby oil wells were analyzed and found to be Na–Ca–Cl type brine with TDS concentrations ranging from 115,000 to 185,000 mg/L. Ratios of major anions and cations in samples from the plume wells were comparable to those of the produced water, but the concentrations in the plume wells were a factor of 6–100 less than the oil well concentrations.
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Fig. 1. Map of the OSPER ‘‘A’’ site. Contours indicate elevation in meters above mean sea level. Sampling well locations are indicated by circles.
3. Site hydrology Twelve completions in 5 augured boreholes (AA02, AA06, AA07, AA10 and AA11) were
instrumented with pressure transducers enabling frequent automatic monitoring of water level in the open wells. Transducer output was recorded once per hour using a data logger. Water levels were
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W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163 A
A’
North
South
230 Pit
AE07
Elevation in meters
225
AE55 AE06
AA04 AA02
AA10
50-100 1820
220
4720
AE51
11,300
2,000
4810 215
Skiatook Lake
AA01
10,800
22,400
9,700
16,100
19,100
10,000 2,000
210
AA06 AA61 AE13
20 15 ,000 ,00 0
1950
29,900
1670
205
26,100
200 0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 2. Salinity values (TDS in mg/L) in wells in 2005 and concentration contours along a south–north transect (A–A 0 ).
A’
Southwest AA07
AA08 AA06 AA61 AE53 AE13 16,900
10,000 10 ,0 00
00 5,000
215
19,100 14,000
11,800 1480
205
16,000
1,710
5,000
14,900 5,690 00 ,0 15
29,900
1,330
210
C’ AA09
220 2,5
Southeast
AA05
AA13
20 ,00 0
Elevation in meters above sea level
C 225
0 ,00 10
2,500
26,100
2,460
2,480
200 0
30
60
90
120
150
180
210
240
Distance along traverse, in meters
219.4
18
219.2
16
219.0
Raw water level data
14
218.8
12
218.6
10 8
218.4 Barometric effects removed
6
218.2 218.0
Daily rainfall total in cm
Water level elevation in meters above sea level
Fig. 3. Salinity measurements (TDS in mg/L) in wells in 2005 and 0 concentration contours along a west–east transect (C–A –C 0 ).
4 Daily rainfall
2
217.8 217.6 1/1/2005
0 4/2/2005
7/2/2005
10/1/2005 12/31/2005
4/1/2006
Date
Fig. 4. Removing barometric effects from well water level data. The ‘‘barometric effects removed’’ data values have been displaced downward 0.3 m for purposes of illustration.
also measured once a month using a hand-held electronic water level tape. A tipping-bucket rain gauge was installed at the site. Each 0.25 mm (0.01 inch) of rainfall resulted in a bucket tip that was recorded by the data logger.
Plots of well water level vs. time show high variability that is inversely correlated with barometric pressure (Fig. 4). This is a problem because the variation in well water levels caused by changing barometric pressure makes it difficult to determine if well water levels also respond to rainfall or other hydrologic events such as lake level rise or nearby well pumping. The barometric effects were removed from the well water level data using multiple-regression deconvolution (Rasmussen and Crawford, 1997). This technique also is a diagnostic tool useful in developing conceptual models of the ground water flow system. The computer program BETCO (http://www. sandia.gov/betco/) developed by Sandia National Laboratories (Toll and Rasmussen, 2007) was used to carry out the deconvolution. Based on the work of Rasmussen and Crawford (1997), BETCO compares well water level data to barometric pressure measurements taken as much as 12 h earlier in order to calculate regression coefficients between barometric pressure change and well water level change as a function of time lag between the measurements. BETCO also calculates a ‘‘residual’’ well water level with barometric effects removed (Fig. 4). The well water level data support the conceptual model that the water-bearing sandstone units at this site are ‘‘confined,’’ and are not well connected to the soil or surface water. Although there were many large and intense rainfall events during the period of record, none of the wells responded to individual rainfall events (e.g. Fig. 4). The barometric response function calculated in the deconvolution analysis also supports the confined aquifer model. BETCO calculates the barometric response function as a function of time lag between barometric pressure change and well water level change (Fig. 5). The barometric response function at a given time lag s is the sum of the calculated regression coefficients between water level and barometric pressure for time lags less than or equal to s. For example, the correlation coefficient between water level in well AA11 (shallow) and simultaneous barometric pressure is 0.49; the correlation coefficient between water level and barometric pressure measured 1 h prior to the water level measurement is 0.12; therefore, the barometric response function for well AA11 (Fig. 5, shallow screen) at time lag = 0 is 0.49, and the barometric response function at time lag = 1 h is 0.61. For all of the wells at this site the barometric response function increases as a function of time lag for time lags less
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163 Table 1 Hydraulic parameters derived from slug tests
1.0
Barometric response function
Medium depth screen (202.7 to 205.4 m elevation) 0.9
0.7
Shallow screen (214.0 to 216.7 m elevation)
0.6
0.5
0.4
Well number
Elevation (m)
Hydraulic conductivity (cm/day)
Storage coefficient
AA06s AA06d AA07d AA08s AA11s AA11d
209.7–212.2 203.0–205.5 202.1–204.9 212.6–215.5 214.0–216.7 198.4–201.0
1.3 0.8 0.3 0.4 1.3 7.1
103 105 106 103 102 106
Deep screen (198.4 to 201 m elevation)
0.8
2159
Designations ‘‘s’’ and ‘‘d’’ refer to ‘‘shallow’’ and ‘‘deep’’ completions, respectively. 0
2
4
6
8
10
12
Time lag in hours
Fig. 5. Barometric response functions for completions in borehole AA11. The ‘‘shallow’’ screen is in stratigraphic Unit 4. Both the ‘‘medium depth’’ and ‘‘deep’’ screens are in stratigraphic Unit 5.
than about 4 h, which is indicative of a confined aquifer response (Rasmussen and Crawford, 1997). The small drop in the barometric response function for time lags greater than 4 h in Fig. 5 is not significant, considering the uncertainty in the water level measurements. On the other hand, a barometrically active well in an unconfined aquifer generally has peak barometric response function at zero time lag, and near zero barometric response function at long lag times (Weeks, 1979). Barometric efficiency, which equals the well barometric response function at long time lags, was very high (75–90%) for all wells at this site. High barometric efficiency indicates that the rocks are not very compressible. A rapid fluctuation in barometric pressure does not result in a significant change in the total water head in the aquifers. On the other hand, the total head in the wells responds immediately to changing barometric pressure. The difference in total head between the wells and the aquifer causes water to move in and out of the wells in response to barometric pressure fluctuations. Slug tests were run using 6 of the large diameter completions in order to estimate hydraulic conductivity (Table 1). Each test was initiated by submerging a solid metal slug into the well in order to rapidly increase the water level in the well by about 13 cm. The rate of fall of the well water level back to the initial condition, which required 1–16 h, was monitored with a pressure transducer. Measured normalized head vs. dimensionless time plots fit very well to the confined aquifer slug test type curves developed by Cooper et al. (1967). Hydraulic conductivity estimated from type curve analysis varied
from 0.3 to 7.0 cm/day over the 6 wells, with an average value of 1.9 cm/day. The slug test results indicate that the hydraulic conductivity is relatively low. Storage coefficient derived from the type curve fits varied from 0.01 to 1 · 106. Larger scale pump test measurements of hydraulic conductivity are probably not practical at this site using existing wells because the pumping rates that can be sustained are too low. Water levels in the wells were generally above lake level, indicating that ground water is flowing toward the lake. An exception is well AA06, which is the larger diameter well closest to the lake (Fig. 1). During the study period the lake surface elevation varied from a high of about 219.3 m above sea level in March 2004 to a low of about 215.3 m in August 2006. The lake operators attempt to maintain the water surface elevation at about 217.7 m (‘‘normal pool’’). However, during floods the lake is allowed to reach higher levels, and the lake backs up into the ephemeral creek bed near well AA06. Under flood conditions, the hydrographs indicate that the aquifer near well AA06 is probably being recharged by lake water. The lake elevation is above the water level in the shallow completion (AA06s), which is above the water level in the deep completion (AA06d), indicating downward flow. Although, there is a time lag, during high lake stage the water head in AA06 eventually becomes approximately equal to the water head in the lake. However, during drought, such as occurred in July 2005 to August 2006, the lake shore retreats tens of meters away from the well, and the head in well AA06 appears to be independent of lake level. Several lines of evidence suggest that ground water recharge from rainfall at this site is low. Although, the wells do not respond rapidly to rainfall, the water head in many of the wells does move slowly up and down in an annual wave pattern (e.g.
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163
Fig. 4). Water levels are minimum in January and maximum in June each year. The average increase in water head in the wells from January to June is about 1 m. One interpretation of this result is that there is ground water recharge in the January to June time period, and drainage thereafter each year in an annual cycle. Assuming the maximum value of storage coefficient (S 0.01), an increase in head (DH) of 1 m in the aquifer indicates a maximum recharge rate of about 1 cm/a (SDH). Long-term meteorological records (http://www.swt-wc.usace.army.mil/SKIA.lakepage.html) indicate average annual potential evapotranspiration (180 cm) at Skiatook Lake is about double average annual precipitation (89 cm). Soils in the salt scar area are clay rich, which tends to seal the soil surface, limiting infiltration and promoting rapid runoff. Surface flow measurements in the ephemeral creek, measured in a weir near well AA06 (Fig. 1), indicate that runoff begins very rapidly after the onset of rain; flow in the creek slows quickly after rain stops, indicating there is little storage capacity in surface sediment or outflow from seeps. Most rainfall events at this site are high intensity, short duration events that promote runoff and limit recharge. Finally, the fact that a subsurface salt plume remains at this site 33 a after the end of significant oil production indicates that recharge and lateral flow of ground water through the plume are limited. 4. Flow and transport modeling In order to explore, compare, and illustrate conceptual models of the site, the finite-difference model STOMP (White and Oostrom, 2000) was used to simulate the subsurface salt plume. The model was used to simulate two-dimensional transport in a ver0 tical slice of the subsurface lying below the A–A transect (Fig. 1). The solution domain was 300 m long by 30 m high. Each finite-difference cell was 5 m wide by 1 m high. The domain was divided into 5 stratigraphic units. The assumed stratigraphy, which is based on Otton et al. (2007), is shown in Fig. 6. Assumed properties of the stratigraphic units are summarized in Table 2. The slope of the ground surface, which averages about 0.035 m/m downward toward the lake, was simulated by designating nodes in the domain above the ground surface to be ‘‘inactive.’’ The upper model boundary was thus adjusted to approximately coincide with the sloping ground surface. Each stratigraphic unit was assumed to have longitudinal and transverse disper-
230
Unit 2 Elevation in meters
2160
Inactive nodes
Unit 3
220
Soil
Unit 4 210
Unit 5 200 0
50
100
150
200
250
300
X-direction node positions in meters
Fig. 6. Model stratigraphy.
Table 2 Summary of assumed hydraulic parameters used in simulations Stratigraphic unit
Thickness (m)
Porosity
Hydraulic conductivity (cm/day)
Soil Unit Unit Unit Unit
1.0 3.0 3.0 6.0 17.0
0.24 0.24 0.10 0.20 0.10
7.0 7.0 1.0 7.0 1.0
2 3 4 5
sivity equal to 100.0 cm and 10.0 cm, respectively. The brine density q was assumed to vary as a function of concentration according to q = qw(1.0 + 6.88e7 · C), where qw is the density of water and C is TDS concentration in mg/L (Kharaka et al., 1988). Assumed boundary conditions were the following: constant head boundaries on the left (upslope) and right edges of the solution domain; uniform recharge flux of 1 cm/a at the ground surface; and no flow boundary at the bottom. Using these boundary conditions, STOMP was run until a steady state solution was obtained that was used as the initial condition prior to the oil production era. The assumed lateral head gradient was 0.035 m/m, which equals the average ground slope. Solute concentration prior to the oil production era was assumed to be zero. During oil production, it was assumed that brine mixed with precipitation and ground water, producing a fluid in the pit having a TDS of 50,000 mg/L. This fluid was assumed to infiltrate through the bottom of the brine pit at a rate of 15.0 cm/a from 1912 to 1973. This rate was chosen because it infiltrates approximately the total mass of salt observed in the plume in 2005 (Kharaka et al., 2005). After 1973, recharge was
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163
2005 was increased about 7% in the variable density case. The modeled concentration distributions compare reasonably well with concentrations measured in 2005 down slope from the brine pit. However, concentrations in well AA10, which is upslope from the brine pit, are anomalously high compared to the model. It is possible that the brine in AA10 came from another source located upslope of the brine pit. It is also possible that some brine moved to the south after infiltrating into the bottom of the brine pit. The hydraulic head in the deep completion TDS concentration (mg/L) in 1973 0 0
0 0
North
South Pit
225
50-100
1820
4720
220
11,300 4810
215
22,400
16,100
9,700 19,100
10,800 29,900
210
1950 1670
205
26,100
200 0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 8. Simulated TDS concentration distribution along the 0 A–A transect in 1973. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
TDS concentration (mg/L) in 1987 0 0
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000 North
South
A’
A
230
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000 North
South
A’
230
Pit 225
Pit 225
50-100
1820
4720
220
11,300 4810
215
22,400
16,100
9,700 19,100
10,800 29,900
210
1950 1670
205
Elevation in meters
Elevation in meters
A’
230
TDS concentration (mg/L) in 1918
A
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000
A
Elevation in meters
assumed to revert to 1.0 cm/a over the entire top boundary. Solute concentration in the recharge water after 1973 was assumed to be zero. The effect of filling Skiatook Lake was simulated by increasing the constant head boundary condition at the right edge of the domain by about 6 m in 1987. Graphs of simulated TDS distribution vs. time (Figs. 7–10) suggest that even though the rocks below the pit have low hydraulic conductivity, slow leakage through the bottom of the pit gradually filled the pore space beneath the pit with brine and created the plume. According to the model, after the site was abandoned the plume migrated slowly down gradient. Model results imply that spreading and natural attenuation of the brine plume caused by mixing with fresh ground water recharge was limited. Because filling the lake raised the hydraulic head at the toe of the plume, lateral flow decreased, and the simulated plume became nearly stagnant after 1987 (Figs. 9 and 10). Model results were very sensitive to the assumed value of hydraulic conductivity. Doubling the assumed hydraulic conductivities resulted in a simulation in which nearly all of the salt was washed out through the right domain boundary by 2005. Inclusion of the effect of TDS concentration on brine density had only a small impact on the simulated concentration distributions. Comparing simulations run with and without varying brine density, the geometry of the simulated plume was essentially unchanged, but the peak modeled concentration in
2161
1820
200
4720
220
11,300 4810
215
22,400
9,700
16,100
19,100
10,800 29,900
210
1950 1670
205
26,100
Skiatook Lake
50-100
26,100
200
0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 7. Simulated TDS concentration distribution along the A– 0 A transect in 1918. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 9. Simulated TDS concentration distribution along the A– 0 A transect in 1987. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
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TDS concentration (mg/L) in 2005 0 0 A
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000 North
South
A’
230 Pit
Elevation in meters
225
Skiatook Lake
50-100
1820
4720
220
11,300 4810
215
9,700
16,100
19,100
29,900
210
1950 1670
205
200
22,400
10,800
26,100
0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 10. Simulated TDS concentration distribution along the 0 A–A transect in 2005. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
of well AA10 is presently below the elevation of the pit, indicating the potential for flow from the pit to well AA10. The subsurface flow paths at this site are more complex than indicated by the simple model. The simulations illustrate a reasonable conceptual model of plume formation and evolution at this site. The plume could have been established throughout Units 2–4 by slow infiltration of salt water over the 60-a production period. Once the site was abandoned, results suggest that recharge and lateral flow of water through the plume were limited, and partly because of the filling of Skiatook Lake, the plume has largely stagnated. 5. Summary and conclusions As a part of a multidisciplinary study of the impact of oil production activities on the local environment, ground water contamination at an abandoned oil production site near Skiatook Lake in Osage County, Oklahoma was investigated. Extensive coring shows that the rocks are interbedded sandstones and shale. Screened wells were installed in 41 boreholes. Well water samples indicate there is a plume of high salinity ground water extending in the subsurface far beyond the area of a surface salt scar at this site. The main dissolved species are Na and Cl, with TDS ranging as high as 30,000 mg/L. Automatic monitoring revealed that water levels in the wells respond rapidly to changes in barometric pressure. A multiple-regression deconvolution analysis was used to filter out baro-
metric effects. The barometric response function indicates the wells are completed in ‘‘confined’’ aquifers. Slug tests run on the wells are consistent with a confined aquifer model. Hydraulic conductivity was low, ranging from 0.3 to 7.0 cm/day. Water level vs. time data indicate that there is no significant change in water level in response to rainfall events, suggesting recharge is limited. On the other hand, water levels moved up and down in an annual cycle that suggests there is some longterm recharge. Only one well, which is quite close to the lake, responded to lake level changes. In order to illustrate and explore conceptual models, a finite-difference flow and transport model of the site was developed. A simple two-dimensional model did a reasonable job of simulating the TDS concentration distribution measured in 2005. Modeling results support the hypothesis that although recharge and hydraulic conductivity are low, production at this site occurred for long enough time for a sufficient amount of brine to leak from pits to fill the pore space beneath the pits and create the plume. According to the model, after the site was abandoned, spreading and natural attenuation of the brine plume caused by mixing with fresh ground water recharge was limited. The model suggests that, partly because of the filling of Skiatook Lake, the plume has largely stagnated. Results indicate that under present conditions the brine plume will exist for many years to come with continued deleterious environmental impact at ground water discharge points. Because hydraulic conductivity is low, remediation by flushing or fluid injection is probably impractical at this site. Conditions at this site are likely to be typical of oil production sites in Oklahoma where underlying rocks are relatively impermeable. Acknowledgements We are grateful to the Osage Indian Nation, to the Army Corps of Engineers and Bureau of Indian Affairs for permission to conduct research at this site. We are grateful also for the financial support for this research provided by DOE National Petroleum Technology Office, E&P Environmental. Reconnaissance Geoprobe drilling was carried out by USEPA’s Ada Research laboratory (Donald Kampbell, supervisor). We thank Mart Oostrom and Mark White of DOE PNNL for invaluable help in implementing the STOMP code. We thank Nathaniel Toll of Sandia National Laboratory for
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163
providing the BETCO code. We thank Jim Otton, George Breit, Jeffrey Hanor and Hedeff Essaid for reviewing earlier versions of this manuscript and suggesting important changes and modifications. References Cooper, H.H., Bredehoeft, J.D., Papadopulos, I.S., 1967. Response of a finite-diameter well to an instantaneous charge of water. Water Resour. Res. 3, 263–269. Kharaka, Y.K., Dorsey, N.S., 2005. Environmental issues of petroleum exploration and production: introduction. Environ. Geosci. 12, 61–63. Kharaka, Y.K., Gunter, W.D., Aggarwal, P.K., Perkins, E.H., DeBraal, J.D., 1988. SOLMINEQ.88: A computer program for geochemical modeling of water-rock interactions: US Geol. Surv. Water-Res. Invest. Rep. 88-4227. Kharaka, Y.K., Thordsen, J.J., Kakouros, E., Herkelrath, W.N., 2005. Impacts of petroleum production on ground and surface water: results from the Osage-Skiatook Petroleum Environmental Research A site, Osage County, Oklahoma. Environ. Geosci. 12, 127–138.
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Otton, J.K., Zielinski, R.A., Smith, B.D., Abbott, M.M., 2007. Geologic controls on movement of produced water releases at US geological survey research site A, Osage county, Oklahoma. Appl. Geochem. 22, 2138–2154. Otton, J.K., Zielinski, R.A., Smith, B.D., Abbott, M.M., Keeland, B.D., 2005. Environmental impacts of oil production on soil, bedrock, and vegetation at the U.S. Geological Survey Osage-Skiatook Petroleum Environmental Research site A, Osage County, Oklahoma. Environ. Geosci. 12, 73–87. Rasmussen, T.C., Crawford, L.A., 1997. Identifying and removing barometric pressure effects in confined and unconfined aquifers. Ground Water 35, 502–511. Sackett, R.L., Bowman, I., 1905. The disposal of strawboard and oil-well wastes. U.S. Geol. Surv. Water-Supply Irrig. Pap. No. 113. Toll, N.J., Rasmussen, T.C., 2007. Removal of barometric pressure effects and earth tides from observed water levels. Ground Water 45, 101–105. Weeks, E.P., 1979. Barometric fluctuations in wells tapping deep unconfined aquifers. Water Resour. Res. 15, 1167–1176. White, M.D., Oostrom, M., 2000. STOMP Subsurface Transport Over Multiple Phases, Version 2.0, Users Guide. PNNL12034, Pacific Northwest National Laboratory, Richland, WA.
Hydrology and subsurface transport of oil-field brine at the U.S. Geological Survey OSPER site ‘‘A’’, Osage County, Oklahoma W.N. Herkelrath
a,*
, Y.K. Kharaka a, J.J. Thordsen a, M.M. Abbott
b
a b
U.S. Geological Survey, Menlo Park, CA 94025, United States U.S. Geological Survey, Oklahoma City, OK 73116, United States Available online 5 May 2007
Abstract Spillage and improper disposal of saline produced water from oil wells has caused environmental damage at thousands of sites in the United States. In order to improve understanding of the fate and transport of contaminants at these sites, the U.S. Geological Survey carried out multidisciplinary investigations at two oil production sites near Skiatook Lake, Oklahoma. As a part of this effort, the hydrology and subsurface transport of brine at OSPER site ‘‘A’’, a tank battery and pit complex that was abandoned in 1973, was investigated. Based on data from 41 new boreholes that were cored and completed with monitoring wells, a large (200 m · 200 m · 20 m) plume of saline ground water was mapped. The main dissolved species are Na and Cl, with TDS in the plume ranging as high as 30,000 mg/L. Analysis of the high barometric efficiency of the wells indicated a confined aquifer response. Well-slug tests indicated the hydraulic conductivity is low (0.3–7.0 cm/day). Simplified flow and transport modeling supports the following conceptual model: (1) prior to the produced water releases, recharge was generally low (1 cm/a); (2) in 60 a of oil production enough saline produced water in pits leaked into the subsurface to create the plume; (3) following abandonment of the site in 1973 and filling of Skiatook Reservoir in the mid1980s, recharge and lateral flow of water through the plume returned to low values; (4) as a result, spreading of the brine plume caused by mixing with fresh ground water recharge, as well as natural attenuation, are very slow. Published by Elsevier Ltd.
1. Introduction Salt water produced as a byproduct of petroleum production can cause long-lasting environmental damage. In the early days of oil production in the United States, oil-field brine was commonly discharged into streams or brine pits (Sackett and Bowman, 1905; Kharaka and Dorsey, 2005). Although, legislation enacted in the 1970s made sur*
Corresponding author. E-mail address: [email protected] (W.N. Herkelrath).
0883-2927/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2007.04.004
face disposal of oil-field brine illegal in most states, the United States has been left with a legacy of thousands of sites that have been impacted by saline water produced from oil and gas wells. In 2001 the U.S. Geological Survey, in cooperation with the U.S. EPA, began a multidisciplinary study of the impact of oil production on the nearfield environment at two oil-field sites near Skiatook Lake in Osage County, Oklahoma. The sites have been dubbed the ‘‘Osage-Skiatook Petroleum Environmental Research’’ (OSPER) sites, ‘‘A’’ and ‘‘B.’’ OSPER site ‘‘A,’’ the focus of this investigation, is
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a legacy site (for location map see Otton et al., 2007, this issue) that started production in 1912, but has been abandoned since about 1973, although production continued elsewhere on the lease. Intensive studies of the geology, geophysics, geochemistry, hydrology and microbiology of the subsurface environment were carried out at this site (Kharaka et al., 2005; Otton et al., 2007). The goal of the investigation was to document and quantify the contaminant plumes found at the site, and to develop a better understanding of the processes that influence the fate and transport of contaminants. This paper summarizes results of hydrologic investigations and includes development of a conceptual and numerical model of subsurface brine transport at the site. 2. Site background A detailed history and description of the site geology is given by Otton et al. (2005, 2007). Geochemistry of the site was discussed by Kharaka et al. (2005). Briefly, about 120,000 barrels of oil were produced from wells near the site in the years 1913–1973. To retrieve the oil, a mixture of oil and brine from nearby wells was pumped into redwood tanks, separated, and then moved in a trench to two unlined earthen pits (Fig. 1). One pit held the oil, which was recovered by the operator. The other pit temporarily held the brine, which was discharged into an ephemeral creek downslope (north) of the pit. Otton et al. (2007) estimate that approximately 190,000 m3 of brine were produced at this site. The brine pit was roughly circular, with a diameter of approximately 15 m. A salt scar approximately 60 m wide by 100 m long north of the pit developed early on and persists in 2006. The northern extent of the scar is currently truncated by Skiatook Lake, which was filled in 1987. Geology at this site was investigated by surface mapping and coring (Otton et al., 2005, 2007). Thin sections were analyzed to estimate porosity. Relatively thin (0–1.0 m thick) surficial sediments composed of eolian sand and colluvium overlie bedrock that consists mainly of sandstone and shale. Otton et al. (2007) identified 4 stratigraphic units in the salt scar area (Units 2–5, increasing in depth from 2 to 5). Unit 2 is comprised of weathered sandstone with average porosity of about 24% and thickness of about 3 m. Unit 3, which is about 3 m thick, contains interbedded thin layers of weathered sandstone and shale. The sandstone in Unit 3 has high porosity (18%), but the shale
porosity is low (1%). Unit 4, which is about 6 m thick, is largely unweathered dolomite-cemented sandstone with variable porosity ranging from 1% to about 20%. Unit 5 is composed of interbedded muddy sandstone, siltstone, mudstone and shale. Sandstones in Unit 5 have about 12% porosity, whereas the other rocks in this unit have a median porosity of about 2%. A total of 41 boreholes were drilled, and screened wells were installed for water sampling, chemical analysis, water level monitoring, and hydraulic testing (Fig. 1). Chemical analyses were published by Kharaka et al. (2005). A Geoprobe rig was used for reconnaissance drilling and to install 25 shallow (<3 m deep), 25-mm-diameter monitoring wells (AE series, Fig. 1). Auger and rotary rigs were used to drill 16, 133-mm-diameter boreholes (AA series, Fig. 1). Two or 3 separate screened wells were installed in each auger hole, enabling multilevel water sampling and head measurements. Completions were placed in water-bearing sandstone units. Each completion consists of a 51-mm-diameter PVC pipe that runs from about 1 m above the surface down to a screened interval at the bottom of the pipe. The borehole around each screen was filled with a layer of coarse sand. Completions were hydraulically isolated from one another by filling the borehole above and below the sand layer with bentonite. Analysis of water samples from the wells indicates there is a large (200 m · 200 m · 20 m) plume of high TDS ground water that extends in the subsurface far beyond the area of the salt scar at the surface. The plume extends from well AA06 to AA10 on a north–south transect (Fig. 2), and from well AA09 to AA05 on an east–west transect (Fig. 3). TDS in the plume ranges as high as 30,000 mg/L, with Na and Cl being the main dissolved species in the contaminated plume water. Water in the plume contrasts with water samples from nearby background ground water wells, which have low salinity (150–1000 mg/L TDS) and have comparable values for the equivalent concentrations of Na, Mg and Ca, as well as for Cl, SO4 and HCO3. Produced water samples from nearby oil wells were analyzed and found to be Na–Ca–Cl type brine with TDS concentrations ranging from 115,000 to 185,000 mg/L. Ratios of major anions and cations in samples from the plume wells were comparable to those of the produced water, but the concentrations in the plume wells were a factor of 6–100 less than the oil well concentrations.
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163
2157
Fig. 1. Map of the OSPER ‘‘A’’ site. Contours indicate elevation in meters above mean sea level. Sampling well locations are indicated by circles.
3. Site hydrology Twelve completions in 5 augured boreholes (AA02, AA06, AA07, AA10 and AA11) were
instrumented with pressure transducers enabling frequent automatic monitoring of water level in the open wells. Transducer output was recorded once per hour using a data logger. Water levels were
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W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163 A
A’
North
South
230 Pit
AE07
Elevation in meters
225
AE55 AE06
AA04 AA02
AA10
50-100 1820
220
4720
AE51
11,300
2,000
4810 215
Skiatook Lake
AA01
10,800
22,400
9,700
16,100
19,100
10,000 2,000
210
AA06 AA61 AE13
20 15 ,000 ,00 0
1950
29,900
1670
205
26,100
200 0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 2. Salinity values (TDS in mg/L) in wells in 2005 and concentration contours along a south–north transect (A–A 0 ).
A’
Southwest AA07
AA08 AA06 AA61 AE53 AE13 16,900
10,000 10 ,0 00
00 5,000
215
19,100 14,000
11,800 1480
205
16,000
1,710
5,000
14,900 5,690 00 ,0 15
29,900
1,330
210
C’ AA09
220 2,5
Southeast
AA05
AA13
20 ,00 0
Elevation in meters above sea level
C 225
0 ,00 10
2,500
26,100
2,460
2,480
200 0
30
60
90
120
150
180
210
240
Distance along traverse, in meters
219.4
18
219.2
16
219.0
Raw water level data
14
218.8
12
218.6
10 8
218.4 Barometric effects removed
6
218.2 218.0
Daily rainfall total in cm
Water level elevation in meters above sea level
Fig. 3. Salinity measurements (TDS in mg/L) in wells in 2005 and 0 concentration contours along a west–east transect (C–A –C 0 ).
4 Daily rainfall
2
217.8 217.6 1/1/2005
0 4/2/2005
7/2/2005
10/1/2005 12/31/2005
4/1/2006
Date
Fig. 4. Removing barometric effects from well water level data. The ‘‘barometric effects removed’’ data values have been displaced downward 0.3 m for purposes of illustration.
also measured once a month using a hand-held electronic water level tape. A tipping-bucket rain gauge was installed at the site. Each 0.25 mm (0.01 inch) of rainfall resulted in a bucket tip that was recorded by the data logger.
Plots of well water level vs. time show high variability that is inversely correlated with barometric pressure (Fig. 4). This is a problem because the variation in well water levels caused by changing barometric pressure makes it difficult to determine if well water levels also respond to rainfall or other hydrologic events such as lake level rise or nearby well pumping. The barometric effects were removed from the well water level data using multiple-regression deconvolution (Rasmussen and Crawford, 1997). This technique also is a diagnostic tool useful in developing conceptual models of the ground water flow system. The computer program BETCO (http://www. sandia.gov/betco/) developed by Sandia National Laboratories (Toll and Rasmussen, 2007) was used to carry out the deconvolution. Based on the work of Rasmussen and Crawford (1997), BETCO compares well water level data to barometric pressure measurements taken as much as 12 h earlier in order to calculate regression coefficients between barometric pressure change and well water level change as a function of time lag between the measurements. BETCO also calculates a ‘‘residual’’ well water level with barometric effects removed (Fig. 4). The well water level data support the conceptual model that the water-bearing sandstone units at this site are ‘‘confined,’’ and are not well connected to the soil or surface water. Although there were many large and intense rainfall events during the period of record, none of the wells responded to individual rainfall events (e.g. Fig. 4). The barometric response function calculated in the deconvolution analysis also supports the confined aquifer model. BETCO calculates the barometric response function as a function of time lag between barometric pressure change and well water level change (Fig. 5). The barometric response function at a given time lag s is the sum of the calculated regression coefficients between water level and barometric pressure for time lags less than or equal to s. For example, the correlation coefficient between water level in well AA11 (shallow) and simultaneous barometric pressure is 0.49; the correlation coefficient between water level and barometric pressure measured 1 h prior to the water level measurement is 0.12; therefore, the barometric response function for well AA11 (Fig. 5, shallow screen) at time lag = 0 is 0.49, and the barometric response function at time lag = 1 h is 0.61. For all of the wells at this site the barometric response function increases as a function of time lag for time lags less
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163 Table 1 Hydraulic parameters derived from slug tests
1.0
Barometric response function
Medium depth screen (202.7 to 205.4 m elevation) 0.9
0.7
Shallow screen (214.0 to 216.7 m elevation)
0.6
0.5
0.4
Well number
Elevation (m)
Hydraulic conductivity (cm/day)
Storage coefficient
AA06s AA06d AA07d AA08s AA11s AA11d
209.7–212.2 203.0–205.5 202.1–204.9 212.6–215.5 214.0–216.7 198.4–201.0
1.3 0.8 0.3 0.4 1.3 7.1
103 105 106 103 102 106
Deep screen (198.4 to 201 m elevation)
0.8
2159
Designations ‘‘s’’ and ‘‘d’’ refer to ‘‘shallow’’ and ‘‘deep’’ completions, respectively. 0
2
4
6
8
10
12
Time lag in hours
Fig. 5. Barometric response functions for completions in borehole AA11. The ‘‘shallow’’ screen is in stratigraphic Unit 4. Both the ‘‘medium depth’’ and ‘‘deep’’ screens are in stratigraphic Unit 5.
than about 4 h, which is indicative of a confined aquifer response (Rasmussen and Crawford, 1997). The small drop in the barometric response function for time lags greater than 4 h in Fig. 5 is not significant, considering the uncertainty in the water level measurements. On the other hand, a barometrically active well in an unconfined aquifer generally has peak barometric response function at zero time lag, and near zero barometric response function at long lag times (Weeks, 1979). Barometric efficiency, which equals the well barometric response function at long time lags, was very high (75–90%) for all wells at this site. High barometric efficiency indicates that the rocks are not very compressible. A rapid fluctuation in barometric pressure does not result in a significant change in the total water head in the aquifers. On the other hand, the total head in the wells responds immediately to changing barometric pressure. The difference in total head between the wells and the aquifer causes water to move in and out of the wells in response to barometric pressure fluctuations. Slug tests were run using 6 of the large diameter completions in order to estimate hydraulic conductivity (Table 1). Each test was initiated by submerging a solid metal slug into the well in order to rapidly increase the water level in the well by about 13 cm. The rate of fall of the well water level back to the initial condition, which required 1–16 h, was monitored with a pressure transducer. Measured normalized head vs. dimensionless time plots fit very well to the confined aquifer slug test type curves developed by Cooper et al. (1967). Hydraulic conductivity estimated from type curve analysis varied
from 0.3 to 7.0 cm/day over the 6 wells, with an average value of 1.9 cm/day. The slug test results indicate that the hydraulic conductivity is relatively low. Storage coefficient derived from the type curve fits varied from 0.01 to 1 · 106. Larger scale pump test measurements of hydraulic conductivity are probably not practical at this site using existing wells because the pumping rates that can be sustained are too low. Water levels in the wells were generally above lake level, indicating that ground water is flowing toward the lake. An exception is well AA06, which is the larger diameter well closest to the lake (Fig. 1). During the study period the lake surface elevation varied from a high of about 219.3 m above sea level in March 2004 to a low of about 215.3 m in August 2006. The lake operators attempt to maintain the water surface elevation at about 217.7 m (‘‘normal pool’’). However, during floods the lake is allowed to reach higher levels, and the lake backs up into the ephemeral creek bed near well AA06. Under flood conditions, the hydrographs indicate that the aquifer near well AA06 is probably being recharged by lake water. The lake elevation is above the water level in the shallow completion (AA06s), which is above the water level in the deep completion (AA06d), indicating downward flow. Although, there is a time lag, during high lake stage the water head in AA06 eventually becomes approximately equal to the water head in the lake. However, during drought, such as occurred in July 2005 to August 2006, the lake shore retreats tens of meters away from the well, and the head in well AA06 appears to be independent of lake level. Several lines of evidence suggest that ground water recharge from rainfall at this site is low. Although, the wells do not respond rapidly to rainfall, the water head in many of the wells does move slowly up and down in an annual wave pattern (e.g.
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163
Fig. 4). Water levels are minimum in January and maximum in June each year. The average increase in water head in the wells from January to June is about 1 m. One interpretation of this result is that there is ground water recharge in the January to June time period, and drainage thereafter each year in an annual cycle. Assuming the maximum value of storage coefficient (S 0.01), an increase in head (DH) of 1 m in the aquifer indicates a maximum recharge rate of about 1 cm/a (SDH). Long-term meteorological records (http://www.swt-wc.usace.army.mil/SKIA.lakepage.html) indicate average annual potential evapotranspiration (180 cm) at Skiatook Lake is about double average annual precipitation (89 cm). Soils in the salt scar area are clay rich, which tends to seal the soil surface, limiting infiltration and promoting rapid runoff. Surface flow measurements in the ephemeral creek, measured in a weir near well AA06 (Fig. 1), indicate that runoff begins very rapidly after the onset of rain; flow in the creek slows quickly after rain stops, indicating there is little storage capacity in surface sediment or outflow from seeps. Most rainfall events at this site are high intensity, short duration events that promote runoff and limit recharge. Finally, the fact that a subsurface salt plume remains at this site 33 a after the end of significant oil production indicates that recharge and lateral flow of ground water through the plume are limited. 4. Flow and transport modeling In order to explore, compare, and illustrate conceptual models of the site, the finite-difference model STOMP (White and Oostrom, 2000) was used to simulate the subsurface salt plume. The model was used to simulate two-dimensional transport in a ver0 tical slice of the subsurface lying below the A–A transect (Fig. 1). The solution domain was 300 m long by 30 m high. Each finite-difference cell was 5 m wide by 1 m high. The domain was divided into 5 stratigraphic units. The assumed stratigraphy, which is based on Otton et al. (2007), is shown in Fig. 6. Assumed properties of the stratigraphic units are summarized in Table 2. The slope of the ground surface, which averages about 0.035 m/m downward toward the lake, was simulated by designating nodes in the domain above the ground surface to be ‘‘inactive.’’ The upper model boundary was thus adjusted to approximately coincide with the sloping ground surface. Each stratigraphic unit was assumed to have longitudinal and transverse disper-
230
Unit 2 Elevation in meters
2160
Inactive nodes
Unit 3
220
Soil
Unit 4 210
Unit 5 200 0
50
100
150
200
250
300
X-direction node positions in meters
Fig. 6. Model stratigraphy.
Table 2 Summary of assumed hydraulic parameters used in simulations Stratigraphic unit
Thickness (m)
Porosity
Hydraulic conductivity (cm/day)
Soil Unit Unit Unit Unit
1.0 3.0 3.0 6.0 17.0
0.24 0.24 0.10 0.20 0.10
7.0 7.0 1.0 7.0 1.0
2 3 4 5
sivity equal to 100.0 cm and 10.0 cm, respectively. The brine density q was assumed to vary as a function of concentration according to q = qw(1.0 + 6.88e7 · C), where qw is the density of water and C is TDS concentration in mg/L (Kharaka et al., 1988). Assumed boundary conditions were the following: constant head boundaries on the left (upslope) and right edges of the solution domain; uniform recharge flux of 1 cm/a at the ground surface; and no flow boundary at the bottom. Using these boundary conditions, STOMP was run until a steady state solution was obtained that was used as the initial condition prior to the oil production era. The assumed lateral head gradient was 0.035 m/m, which equals the average ground slope. Solute concentration prior to the oil production era was assumed to be zero. During oil production, it was assumed that brine mixed with precipitation and ground water, producing a fluid in the pit having a TDS of 50,000 mg/L. This fluid was assumed to infiltrate through the bottom of the brine pit at a rate of 15.0 cm/a from 1912 to 1973. This rate was chosen because it infiltrates approximately the total mass of salt observed in the plume in 2005 (Kharaka et al., 2005). After 1973, recharge was
W.N. Herkelrath et al. / Applied Geochemistry 22 (2007) 2155–2163
2005 was increased about 7% in the variable density case. The modeled concentration distributions compare reasonably well with concentrations measured in 2005 down slope from the brine pit. However, concentrations in well AA10, which is upslope from the brine pit, are anomalously high compared to the model. It is possible that the brine in AA10 came from another source located upslope of the brine pit. It is also possible that some brine moved to the south after infiltrating into the bottom of the brine pit. The hydraulic head in the deep completion TDS concentration (mg/L) in 1973 0 0
0 0
North
South Pit
225
50-100
1820
4720
220
11,300 4810
215
22,400
16,100
9,700 19,100
10,800 29,900
210
1950 1670
205
26,100
200 0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 8. Simulated TDS concentration distribution along the 0 A–A transect in 1973. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
TDS concentration (mg/L) in 1987 0 0
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000 North
South
A’
A
230
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000 North
South
A’
230
Pit 225
Pit 225
50-100
1820
4720
220
11,300 4810
215
22,400
16,100
9,700 19,100
10,800 29,900
210
1950 1670
205
Elevation in meters
Elevation in meters
A’
230
TDS concentration (mg/L) in 1918
A
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000
A
Elevation in meters
assumed to revert to 1.0 cm/a over the entire top boundary. Solute concentration in the recharge water after 1973 was assumed to be zero. The effect of filling Skiatook Lake was simulated by increasing the constant head boundary condition at the right edge of the domain by about 6 m in 1987. Graphs of simulated TDS distribution vs. time (Figs. 7–10) suggest that even though the rocks below the pit have low hydraulic conductivity, slow leakage through the bottom of the pit gradually filled the pore space beneath the pit with brine and created the plume. According to the model, after the site was abandoned the plume migrated slowly down gradient. Model results imply that spreading and natural attenuation of the brine plume caused by mixing with fresh ground water recharge was limited. Because filling the lake raised the hydraulic head at the toe of the plume, lateral flow decreased, and the simulated plume became nearly stagnant after 1987 (Figs. 9 and 10). Model results were very sensitive to the assumed value of hydraulic conductivity. Doubling the assumed hydraulic conductivities resulted in a simulation in which nearly all of the salt was washed out through the right domain boundary by 2005. Inclusion of the effect of TDS concentration on brine density had only a small impact on the simulated concentration distributions. Comparing simulations run with and without varying brine density, the geometry of the simulated plume was essentially unchanged, but the peak modeled concentration in
2161
1820
200
4720
220
11,300 4810
215
22,400
9,700
16,100
19,100
10,800 29,900
210
1950 1670
205
26,100
Skiatook Lake
50-100
26,100
200
0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 7. Simulated TDS concentration distribution along the A– 0 A transect in 1918. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 9. Simulated TDS concentration distribution along the A– 0 A transect in 1987. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
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TDS concentration (mg/L) in 2005 0 0 A
3000 6000 6000 9000 9000 12000 12000 15000 15000 18000 18000 21000 21000 24000 24000 27000 27000 30000 30000 3000 North
South
A’
230 Pit
Elevation in meters
225
Skiatook Lake
50-100
1820
4720
220
11,300 4810
215
9,700
16,100
19,100
29,900
210
1950 1670
205
200
22,400
10,800
26,100
0
25
50
75
100
125
150
175
200
225
Distance along traverse, in meters
Fig. 10. Simulated TDS concentration distribution along the 0 A–A transect in 2005. Numbers indicate measured TDS concentrations for well water samples obtained in 2005.
of well AA10 is presently below the elevation of the pit, indicating the potential for flow from the pit to well AA10. The subsurface flow paths at this site are more complex than indicated by the simple model. The simulations illustrate a reasonable conceptual model of plume formation and evolution at this site. The plume could have been established throughout Units 2–4 by slow infiltration of salt water over the 60-a production period. Once the site was abandoned, results suggest that recharge and lateral flow of water through the plume were limited, and partly because of the filling of Skiatook Lake, the plume has largely stagnated. 5. Summary and conclusions As a part of a multidisciplinary study of the impact of oil production activities on the local environment, ground water contamination at an abandoned oil production site near Skiatook Lake in Osage County, Oklahoma was investigated. Extensive coring shows that the rocks are interbedded sandstones and shale. Screened wells were installed in 41 boreholes. Well water samples indicate there is a plume of high salinity ground water extending in the subsurface far beyond the area of a surface salt scar at this site. The main dissolved species are Na and Cl, with TDS ranging as high as 30,000 mg/L. Automatic monitoring revealed that water levels in the wells respond rapidly to changes in barometric pressure. A multiple-regression deconvolution analysis was used to filter out baro-
metric effects. The barometric response function indicates the wells are completed in ‘‘confined’’ aquifers. Slug tests run on the wells are consistent with a confined aquifer model. Hydraulic conductivity was low, ranging from 0.3 to 7.0 cm/day. Water level vs. time data indicate that there is no significant change in water level in response to rainfall events, suggesting recharge is limited. On the other hand, water levels moved up and down in an annual cycle that suggests there is some longterm recharge. Only one well, which is quite close to the lake, responded to lake level changes. In order to illustrate and explore conceptual models, a finite-difference flow and transport model of the site was developed. A simple two-dimensional model did a reasonable job of simulating the TDS concentration distribution measured in 2005. Modeling results support the hypothesis that although recharge and hydraulic conductivity are low, production at this site occurred for long enough time for a sufficient amount of brine to leak from pits to fill the pore space beneath the pits and create the plume. According to the model, after the site was abandoned, spreading and natural attenuation of the brine plume caused by mixing with fresh ground water recharge was limited. The model suggests that, partly because of the filling of Skiatook Lake, the plume has largely stagnated. Results indicate that under present conditions the brine plume will exist for many years to come with continued deleterious environmental impact at ground water discharge points. Because hydraulic conductivity is low, remediation by flushing or fluid injection is probably impractical at this site. Conditions at this site are likely to be typical of oil production sites in Oklahoma where underlying rocks are relatively impermeable. Acknowledgements We are grateful to the Osage Indian Nation, to the Army Corps of Engineers and Bureau of Indian Affairs for permission to conduct research at this site. We are grateful also for the financial support for this research provided by DOE National Petroleum Technology Office, E&P Environmental. Reconnaissance Geoprobe drilling was carried out by USEPA’s Ada Research laboratory (Donald Kampbell, supervisor). We thank Mart Oostrom and Mark White of DOE PNNL for invaluable help in implementing the STOMP code. We thank Nathaniel Toll of Sandia National Laboratory for
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providing the BETCO code. We thank Jim Otton, George Breit, Jeffrey Hanor and Hedeff Essaid for reviewing earlier versions of this manuscript and suggesting important changes and modifications. References Cooper, H.H., Bredehoeft, J.D., Papadopulos, I.S., 1967. Response of a finite-diameter well to an instantaneous charge of water. Water Resour. Res. 3, 263–269. Kharaka, Y.K., Dorsey, N.S., 2005. Environmental issues of petroleum exploration and production: introduction. Environ. Geosci. 12, 61–63. Kharaka, Y.K., Gunter, W.D., Aggarwal, P.K., Perkins, E.H., DeBraal, J.D., 1988. SOLMINEQ.88: A computer program for geochemical modeling of water-rock interactions: US Geol. Surv. Water-Res. Invest. Rep. 88-4227. Kharaka, Y.K., Thordsen, J.J., Kakouros, E., Herkelrath, W.N., 2005. Impacts of petroleum production on ground and surface water: results from the Osage-Skiatook Petroleum Environmental Research A site, Osage County, Oklahoma. Environ. Geosci. 12, 127–138.
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Otton, J.K., Zielinski, R.A., Smith, B.D., Abbott, M.M., 2007. Geologic controls on movement of produced water releases at US geological survey research site A, Osage county, Oklahoma. Appl. Geochem. 22, 2138–2154. Otton, J.K., Zielinski, R.A., Smith, B.D., Abbott, M.M., Keeland, B.D., 2005. Environmental impacts of oil production on soil, bedrock, and vegetation at the U.S. Geological Survey Osage-Skiatook Petroleum Environmental Research site A, Osage County, Oklahoma. Environ. Geosci. 12, 73–87. Rasmussen, T.C., Crawford, L.A., 1997. Identifying and removing barometric pressure effects in confined and unconfined aquifers. Ground Water 35, 502–511. Sackett, R.L., Bowman, I., 1905. The disposal of strawboard and oil-well wastes. U.S. Geol. Surv. Water-Supply Irrig. Pap. No. 113. Toll, N.J., Rasmussen, T.C., 2007. Removal of barometric pressure effects and earth tides from observed water levels. Ground Water 45, 101–105. Weeks, E.P., 1979. Barometric fluctuations in wells tapping deep unconfined aquifers. Water Resour. Res. 15, 1167–1176. White, M.D., Oostrom, M., 2000. STOMP Subsurface Transport Over Multiple Phases, Version 2.0, Users Guide. PNNL12034, Pacific Northwest National Laboratory, Richland, WA.