In situ and laboratory toxicity of coalbed natural gas produced waters with elevated sodium bicarbonate

In situ and site water experiments

In situ experiments to investigate the acute toxicity of produced waters in the Powder River Structural Basin, Wyoming and Montana, USA, were completed during July–August 2006 and July 2007. Early lifestage fathead minnow and pallid sturgeon (Scaphirhynchus albus; endemic species used in the laboratory experiments 9) were studied during 2006 and 2007, respectively. An additional static renewal experiment with pallid sturgeon was completed in controlled conditions with site-collected water in 2007. In situ experiments were not conducted in the Tongue River because of logistical problems created by the long distances between experimental sites.

Experiments lasted in situ for 96 h. Fish were held in 450-mL cellulose acetate butyrate containers with 149-µm mesh screen covering approximately 40% of the surface area. Each container held 10 fish, and 4 replicate containers were deployed per site. In situ containers were held in place at each site by strapping the individual containers to a vinyl-coated metal dish drying rack that was placed in a perforated (118 L) Rubbermaid high-density polyethylene storage tub. Storage tubs were held in place with stakes, rocks, and rope to prevent loss from potential flash flooding. At 1 site, the depth was not adequate to allow the use of the storage tub, and the dish drying rack was placed directly in the stream channel with a storage tub cover over the top to provide shade.

Experiments were initiated with 2 dph fathead minnow on 26–29 July (experiment 1) and 6 dph fathead minnow on 30 July–02 August, 2006 (experiment 2; “2 dph” and “6 dph” will be used from this point). Newly hatched fish were shipped directly to Buffalo, Wyoming, USA, from Aquatic Biosystems, Fort Collins, Colorado, USA. The brood stock were deemed free of disease and kept in isolation at the Colorado facility. For experiment 1, 2 reference sites and 4 experimental sites were used; Powder River at Moorhead and Clear Creek (reference sites), SA Creek, Upper Beaver Creek, Lower Beaver Creek, and Burger Draw (Figure 1).

After the completion of the 96-h experiments with 2 dph fish, experiment 2 was initiated with 6 dph fathead minnow in situ at Clear Creek (reference site) Upper Beaver Creek, Lower Beaver Creek, and Burger Draw (experimental sites) with fish from the same source.

Survival was monitored every 24 h, and water samples were collected daily from all sites. Water samples were placed on ice and filtered each evening with a 0.45-µm Millipore filter. Subsamples for cation analyses were acidified to less than pH 2.0 with ultrapure nitric acid and were held at 4 °C during storage and shipment for analyses. Anion samples were collected into bicarbonate rinsed containers and stored at 4 °C during storage and shipment. Alkalinity, hardness 10, 11, and pH were measured each evening (Table 1). Water temperature, conductivity, and dissolved oxygen were measured manually at each site at least once daily, and monitored remotely with YSI Data Sonde loggers at Powder River at Moorhead, Clear Creek, SA Creek, and Lower Beaver Creek every 30 min. Mean temperatures (with standard error in parentheses) were recorded as follows Powder River at Moorehead 27.4 °C (2.3 °C), Clear Creek 21.6 °C (3.2 °C), Lower Beaver 21.9 °C (3.4 °C), Upper Beaver 20.3 °C (2.4 °C), Burger Draw 21.9 °C (4.7 °C), and SA Creek 27.6 °C (2.5 °C). Minimum and maximum temperatures recorded at 4 sites with data loggers were Powder River at Moorhead 21.3 °C to 31.2 °C, Clear Creek 15.1 °C to 29.5 °C, Lower Beaver 14.1 °C to 32.0 °C, and SA Creek 16.2 °C to 30.0 °C. Mean dissolved oxygen was 7.6 mg/L or higher at all sites measured with hand-held meters, and data loggers did not reveal measurements less than 5.0 mg/L. Mean conductivity was 857 µS/cm at Clear Creek and 3616 µS/cm at Powder River at Moorhead, 1 of the 2 reference sites. Mean conductivity at the experimental sites were Lower Beaver 2843 µS/cm, Upper Beaver 2612 µS/cm, Burger Draw 3649 µS/cm, and SA Creek 2620 µS/cm. Mean pH ranged from 8.3 at Powder River at Moorhead to 9.3 at Lower Beaver. Additional water chemistry characterization can be found in Farag and Harper 12. Ammonia was measured onsite daily with a Hach test kit (Table 1). Additional trace metal scans were performed on 1 subsample of water from each site (data not presented, concentrations were below chronic criteria established for the protection of aquatic life). In 2007, 2 dph pallid sturgeon were transported from the Montana Fish Wildlife and Parks Miles City, Montana Fish Hatchery, to Buffalo, Wyoming. Pallid sturgeon at 2 dph, 4 dph, and 6 dph were exposed in experiments at Clear Creek (reference site), Upper Beaver Creek, Lower Beaver Creek, and Burger Draw (experimental sites) with the same methods employed with fathead minnow during 2006. The water quality monitoring design and methods were the same as described for our 2006 in situ experiments (Table 2, trace metal scans not presented); however, in addition to performing onsite ammonia assays with a Hach test kit, water samples were collected and acidified for ammonia analyses in the laboratory with the US Environmental Protection Agency 350.1 methodology 13.

In addition to the in situ experiments completed in the field in 2007, a simultaneous experiment was completed with site water in more controlled laboratory conditions. Water was collected at Clear Creek (reference site), Upper Beaver Creek, Lower Beaver Creek, and Burger Draw (experimental sites) and held in insulated 22 L containers. Four replicates of 5 fish were exposed to water from each site in 530 mL containers. The containers were kept in a water bath that fluctuated slightly with the room temperature (16 °C–22 °C), and water in the containers was replaced every 12 h.

Water samples intended for laboratory analyses were kept refrigerated in Buffalo, Wyoming, USA, and transported in ice to the US Geological Survey, Columbia Environmental Research Center, Jackson Field Research Station, Jackson, Wyoming (herein referred to as Jackson Field Research Station). Samples were then sorted and shipped in ice to the State of Montana Department of Health and Human Services Environmental Laboratory for anion and cation analyses. For both experiments in 2006, the survival data were analyzed with the Toxstat software package 14. A t test was performed between the reference sites (experiment 1). Because no difference was observed, the data were pooled and a one-way analysis of variance was performed followed by a Bonferroni means comparison. Because 1 reference site was used during experiment 2, no pooling was necessary. To compare the survival of 2 dph against 6 dph fathead minnow at sites t tests were used. All data met homogeneity and normality assumptions without transformations. In 2007, a one-way analysis of variance was performed followed by a Bonferroni means comparison for 4 dph and 6 dph pallid sturgeon in situ, and 4 dph pallid sturgeon static-renewal experiments. All means were tested with a statistical criterion of p < 0.05.

Potential differences in mean ammonia concentrations derived with field assays of chilled samples and the EPA method 350.1 laboratory assays 13 were defined with t tests in the Toxstat software package with a statistical criterion of p < 0.05. No transformations were necessary because all data met homogeneity and normality assumptions without transformation.

Mixing zone and site water experiments

Mixing zones were characterized at 3 locations of the Powder/Tongue River watersheds: downstream from the confluence of Beaver Creek and the Powder River; downstream from the untreated produced water diffuser in the Tongue River near Decker, Montana; and downstream from the treated produced water diffuser (Higgins Loop Treatment, an ion exchange) near Decker, Montana. These sites were chosen to characterize 3 types of discharge: no treatment of a stream that often contained 100% effluent, no treatment but a diffuser was used to enhance mixing, and water that was treated before discharge. These sites vary from the in situ experiments that were designed to define survivability in tributaries that primarily contained produced water.

Rhodamine WT dye was added at a constant drip into Beaver Creek and into the treated and untreated produced water. At the Powder River, transects were spaced at 4 m, 8 m, 16 m, 32 m, and 804 m below the confluence of Beaver Creek. At the Tongue River, transects were spaced 15 m, 30 m, 60 m, 90 m, and 1200 m downstream from the diffusers for treated and untreated discharge. The dye concentration was measured in the water at evenly spaced locations (0.15 m–3 m) across the river, and spacing was dependent on the width of the river. The number of locations across the width ranged from 5 to 11 locations, and the width of the river at site locations ranged from 3 m to 33.4 m. At each point along the transect, dye concentrations were measured with a YSI–600 oms sonde, equipped with a 6130 Rhodamine WT optical probe. The concentration of the injected dye solution was measured in reference to a dilution series of the stock solution 15.

Toxicity experiments were performed to assess the toxicity of waters from the mixing zone. Site water experiments were completed with water collected from the Powder River and from Beaver Creek near the confluence with the Powder River, in Johnson County, Wyoming. Water also was collected from the Tongue River and 2 diffuser discharge points into the Tongue River in Big Horn County, Montana; 1 site contained treated (Higgins Loop ion exchange) CBNG-produced water, the other site contained untreated CBNG-produced water; water was collected immediately above their discharge point into the Tongue River. Water collected from all sites was placed in acid-washed, 20-L high-density polyethylene containers, which were chilled and transported to the Jackson Field Research Station. The waters were used to complete 96-h static renewal experiments with 2 dph fathead minnow from September to October 2007.

Newly hatched fathead minnow were shipped directly to the Jackson Field Research Station from Aquatic Biosystems, Fort Collins, Colorado, USA. At the Jackson Field Research Station, fish were acclimatized to Tongue River water and held at 20 °C.

Static-renewal experiments with the site waters were completed in 1-L glass beakers filled with 750 mL site water and maintained in a water bath at 20 °C. The fish were exposed for 96 h, and the experimental water was replaced every 24 h; the replacement water was allowed to equilibrate to the exposure temperature in the water bath overnight. Water from Beaver Creek, untreated CBNG-produced water, and treated CBNG-produced water were experimented with at 100%, 75%, and 50% concentrations. Beaver Creek water was diluted with Powder River water to prepare the proper concentrations, and the produced waters were diluted with Tongue River water to prepare the proper concentrations. Waters collected from the Powder and Tongue Rivers served as reference waters for the experiments. Water temperature and dissolved oxygen were monitored twice daily, and water samples were collected daily for water chemistry from the 100% (undiluted) and 50% (diluted) treatments (Table 3).

In situ experiments

Concentrations of NaHCO₃ were elevated at all experimental sites except the reference sites (856–3504 mg/L; Tables 1 and 2). In 2006, ammonia concentrations were largest at Burger Draw and averaged 1.5 mg/L to 2.0 mg/L in 2006, and 1.17 mg/L in 2007. Ammonia concentrations did not exceed 0.5 mg/L at any other sites. Ammonia concentrations in samples measured in the field were greater than acidified samples measured in the laboratory. Mean total ammonia (mg/L) measured 0.01 mg/L at Clear Creek for both laboratory and field, 0.1 mg/L laboratory and 0.4 mg/L field for Lower Beaver, 0.12 mg/L laboratory and 0.29 mg/L field for Upper Beaver, and 1.05 mg/L laboratory and 1.17 mg/L field for Burger Draw. Differences in ammonia concentrations were not significant between the methodologies at Burger Draw, Lower Beaver, and Clear Creeks; however, differences were significant at Lower Beaver Creek (p < 0.05).

Survival of 2 dph fathead minnow in 2006 was 77% and 78% in the Powder River at Moorhead and the Clear Creek sites, respectively. Because no significant difference in survival was seen between the 2 sites, the data were combined to create a pooled reference (78% as pooled reference) for further comparisons. Survival was significantly less at all experimental sites (p ≤ 0.05) when compared with the pooled reference (Table 4) during experiment 1. Survival of 6 dph fathead minnow was 90% at Clear Creek (reference site) and was not significantly different at any experimental site.

Survival of 2 dph pallid sturgeon was low at all sites including the reference in 2007, and the experiment was terminated at 24 h. The experiment was restarted with 4 dph fish, and survival at Clear Creek (reference site) was 95%, whereas survival in all experimental sites was 0% (Table 5). Survival of 4 dph pallid sturgeon in a static-renewal experiment was similar to the in situ experiment, with reduced survival of fish exposed to produced water (0% to 15%), and 90% survival of fish held in Clear Creek reference water. An experiment completed with 6 dph fish provided similar results when compared with the 4 dph fathead minnow, although time to death was longer with the older fish (data not presented).

Site water/mixing zone experiments

In the Powder River at the Beaver Creek confluence, dye concentrations, which represented the portion of the stream composed of Beaver Creek discharge, were greatest on the bank closest to the input source. At the first transect, dye concentrations ranged from 122 µg/L near the inflow of Beaver Creek to 0 µg/L on the opposite stream bank. At 16 m below the confluence of Beaver Creek, dye concentrations across the width of the river ranged from 48 µg/L to 4.4 µg/L, and at 804 m below the confluence of Beaver Creek dye concentrations ranged from 23 µg/L to 7 µg/L.

At the untreated water outflow into Tongue River near Decker, Montana, dye concentrations 15 m downstream from the diffuser ranged from 55 µg/L to 0 µg/L, and were most concentrated in measurements taken closest to the diffuser, whereas 3 measurements located farthest from the diffuser outfall contained little or no dyed produced water. At 30 m, dye ranged from 39 µg/L to 0 µg/L across the river; at 60 m, dye concentrations ranged from 25 µg/L to 0 µg/L; and at 1200 m, the dye ranged from 19 µg/L to 6 µg/L.

At the treated water outflow, dye concentrations measured 15 m downstream from the diffuser ranged from 30 µg/L to 0 µg/L, and the dye was most concentrated in the center of the stream channel near the diffuser outfall. Points along the width of the river that were nearest to both stream banks contained little or no dyed produced water. At 30 m, dye concentrations across the width of the Tongue River ranged from 37 µg/L to 0.6 µg/L, and at 1200 m, dye concentrations ranged from 12 µg/L to 9 µg/L.

Survival of 2 dph fathead minnow exposed to 100% Beaver Creek water and 100% untreated produced water was reduced significantly when compared with Powder River and Tongue River reference sites. Survival of all other mixtures was not significantly different from the reference sites (Figure 2).

Ammonia concentrations were below US Environmental Protection Agency (USEPA) water-quality criteria for all exposures and dilutions except the untreated CBNG-produced water (Table 3) 16. However, ammonia concentrations were sufficient in the untreated produced water (mean = 2.0 mg/L) to potentially contribute to mortality.