Toxic contributions of specific drilling mud components to larval shrimp and crabs

Marine Environmental Research 12 (1984) 45-62

Toxic Contributions of Specific Drilling Mud Components to Larval Shrimp and Crabs

Mark G. Carls & Stanley D. Rice

Northwest and Alaska Fisheries Center, Auke Bay Laboratory, National Marine Fisheries Service, NOAA, PO Box 155, Auke Bay, AK 99821, USA

(Received: 14 November, 1983)

ABSTRACT

We investigated the toxicities of six drilling muds, toxicities of mud fractions (supernatants and suspensions) and the toxicities of common mud components--barite and bentonite (particulates) and ferrochrome lignosulfonate (soluble)--to the stage I larvae of six species of shrimp and crab. The drilling muds we tested were not very toxic to these larvae: LCso's for supernatants ranged from 0"6 to 82 % (vol/vol). Shrimp larvae were slightly more sensitive than crab larvae.

Drilling muds were not rapidly toxic, in contrast to toxicants such as the water-soluble fractions o foil. Supernatants, prepared by centrifuging whole muds, were mildly toxic. Suspensions were more toxic than supernatants and toxicity was greatest when particulates remained suspended: for example, used Cook Inlet mud suspensions were about seven times more toxic than supernatants. The toxicity of used Cook Inlet mud was therefore primarily due to suspended solids (88 %) rather than chemical toxicity: ferrochrome lignosulfonate was relatively toxic alone, but accounted for only about 6 % of the toxicity of used Cook Inlet mud suspensions. Contributions of particulates to mud toxicities varied considerably. Barite and bentonite were not very toxic when tested alone. The toxicity of one mud was caused by its high alkalinity.

I N T R O D U C T I O N

The number ofoi l wells being drilled offshore of Alaska is increasing. One potent ia l source of pollution from oil wel ls is the m u d used during

45 Marine Environ. Res. 0141-1136/84/$03.00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

46 Mark G. Carls, Stanley D. Rice

drilling. Drilling muds bring cuttings to the surface, control subsurface pressures, support the walls of the well hole, suspend the drill string and casing, prevent corrosion and cool and lubricate the bit (IMCO Services, 1980). There are many types of mud with hundreds of possible constituents. Specific mud constituents and their proportions vary considerably, depending on drilling conditions (depth, rock formation, etc.) (Tagatz et al., 1978). Although some mud is discharged during drilling, most is recycled. After a well is completed, however, large quantities of drilling mud may be dumped into the ocean, possibly damaging the planktonic community.

Toxicity tests with drilling muds have generally indicated that drilling muds are not very toxic. For example, adult salmon and shrimp are relatively tolerant to Cook Inlet muds (Dames & Moore, 1978). However, concern about the toxicity of drilling muds still exists, because the toxicities of the many drilling mud additives have not been sufficiently tested on potentially sensitive life stages such as planktonic larvae. Furthermore, sensitive crustacean larvae may be harmed by very short exposures to some toxicants (Rice et al., 1981), and may be equally sensitive to drilling muds or mud additives.

The objective of our study was to determine the toxicity of several drilling muds to sensitive larval stages of shrimp and crab, and to determine which mud fractions and components were responsible for any observed toxicity. In this study, we analyzed the toxicities of two general categories of mud components--supernatants and particulates--and also tested the toxicities of some common components of drilling muds-- ferrochrome lignosulfonate (a water-soluble component), barite and bentonite (two water-insoluble components)--and pH effects.

We tested the toxicities of six different drilling muds during the course of the study: two from Prudhoe Bay and four from Cook Inlet. Four of the muds sampled were collected from various depths during active drilling and two were unused muds. The relative toxicities of the muds gave us clues to the nature of mud component toxicity.

We tested larval animals because larval forms tend to be much more sensitive than the adults (Moore & Dwyer, 1974). Our test organisms were the larvae of three commercially important crab species (king crab (Paralithodes camtschatica), Tanner crab (Chionoecetes bairdi) and Dungeness crab (Cancer magister), two commercially important shrimp species (coonstripe shrimp (Pandalus hypsinotus) and dock shrimp (Pandalus danae)), and the kelp shrimp (Eualus suckleyi). All these

Toxicities of mud components to larval shrimp and crabs 47

species are very sensitive to other toxicants (such as crude oil; Brodersen et al. , 1977; Mecklenburg et al. , 1977). We also compared the relative tolerances of these larvae.

METHODS AND MATERIALS

We tested the toxicity of six water-based drilling muds (Table 1). All of the muds contained ferrochrome lignosulfonate except new Prudhoe Bay mud. Mud composition and physical appearances varied widely. Each mud was stored in a 19-liter bucket at approximately 5 °C. Immediately before the muds were tested, they were mixed in the storage buckets with a barrel mixer at 3450 rpm for a minimum of ½h to ensure sample homogeneity. All assays were completed within 3 months after the muds were collected.

Gravid shrimp and crabs were collected in southeastern Alaska and isolated by species in flow-through tanks partitioned with 363/~ plankton net. Larvae were collected daily after they were released by the captive females.

We used 96-h static tests throughout the study followed by 2 days of additional observations. After preliminary tests using a logarithmic dose series, we often selected a narrower range of concentrations centred

TABLE 1 Description of Drilling Muds used in Tests

Location Well Density i00 % Supernatant depth of mud (m) (kg/liter) pH Salinity Absorbance ~

(%0) due to FCL

Prudhoe Bay (new) - - 1.18 13.2 3-1 b Prudhoe Bay (used) 2926 1.12 8.9 1-8 2.68 Cook Inlet (used) 3 382 1-66 8-6 17.5 173.8 Homer (new) - - 1-17 9.9 5.2 5.46 Homer (used 'spud 'c) - - 1-17 9.9 5-2 5.26 Homer (used) 442 1.07 12.2 4'0 4" 19

°Calculated 100~ values at 278nm. Absorbance at 278+ l nm is ferrochrome iignosulfonate. b NO absorbance peak at 278 nm. c Mud newly placed in production.


around the preliminary LCs0, which was designed to produce as many partial responses as practical. A minimum of six concentrations per test (including controls) were used with 3-5 replicates per concentration and 6-10 larvae per replicate. Maximum biomass did not exceed 0.2 g/liter. Each test began with Stage I larvae 0-3 days old. Tests were not extended through the first molt because of high incidences of natural molting mortality. Although larvae were not fed during the experiments, control mortalities remained low (2 _+ l ~o, but rose to 6 _+ 4 % at 144 h). During the study, water temperature was 5-6 _ 0-8 °C, and salinities ranged from 28.4 to 30-9%0. (In preliminary tests, salinities between 15.1 and 30"9%0 did not affect sensitivities of larvae to mud.)

The relative tolerances of the larvae were compared by exposing them to suspensions and supernatants of used Cook Inlet mud. This mud was selected because it was believed to be representative of used muds commonly in use.

In other tests, toxicities of drilling muds were compared from results of tests in which larvae of king crab and coonstripe shrimp were exposed to either mud suspensions or supernatants of the muds. The suspensions contained both dissolved chemicals and particulate matter: the supernatants contained primarily dissolved chemicals and little or no particulate matter.

We identified dead larvae on the basis of body posture, color and opacity. (Death was difficult to determine accurately in tests with some mud suspensions because larvae were buried.) Larvae were classified as not swimming if they could not maintain their position in the water column or did not swim when mildly stimulated.

Tests with suspended mud

Drilling mud was added to seawater in test tubes to prepare 50 ml suspensions: concentrations ranged from 0 (control) to 10~o mud (vol/vol): a typical dose series was 0.5, l, 3, 5, 7 and l0 ~o. The tubes were capped and shaken vigorously for 10-20 s, then the larvae were added by pipette. Most of the particulates settled out within 1-2 h.

The procedures for testing suspensions of used Cook Inlet mud were slightly different from tests with suspensions of other muds because particulates in used Cook Inlet mud remained in suspension throughout the 6-day assays. Used Cook Inlet mud was added by syringe to 200 ml of seawater (in 400-ml beakers) and agitated: concentrations ranged from 0


to 10 ~ (vol/vol). The larvae, confined in 2 × 15 cm glass tubes with the bottoms screened with 210p plankton net, were then placed in the beakers. Because the suspensions were too opaque for direct observation, we observed the larvae (including controls) by briefly transferring the glass tubes to clean seawater.

Tests with supernatants

To prepare a supernatant for each mud, we added equal volumes of mud and seawater to an Erlenmeyer flask and agitated the flask vigorously for 2 min. This mixture was centrifuged in 30-ml tubes at 12 000 g for l0 min, then aspirated through Whatman No. 5 filters. The supernatant of used Cook Inlet mud was centrifuged again at 54 000 g for l0 min to remove remaining particles. The supernatants may have contained a very small amount of particulate matter in the micron-submicron range.

The salinity of the supernatant was measured and adjusted to 30%o by adding NaCI brine. Fresh supernatants were prepared at the beginning of each experiment. The supernatants were chemically stable, but water condensation from the air caused approximately 0.6 ~ (vol/vol) dilution per day.

Test concentrations of supernatants ranged from 0 (control) to 50 ~o (vol/vol): a typical dose series was 0, 5, 10, 15, 20, 30, and 50 ~ . Larvae, confined in glass tubes with screened ends, were placed in 200 ml of test solution. If any of the highest concentrations of supernatant were too opaque for observation, larvae in all concentrations (including controls) were placed briefly in clean seawater I for observation.

Tests with specific components

Toxicities of each of three components commonly found in the muds (barite, bentonite and ferrochrome lignosulfonate) were determined for larvae of Dungeness crab and dock shrimp. Procedures for tests with barite and bentonite (insoluble particulates) were similar to tests with mud suspensions. Samples of barite or bentonite were weighed and added to test tubes containing 50 ml of seawater. The suspensions were then shaken vigorously. Test concentrations of barite ranged from 0 (control) to 200g/liter, in about 40g/liter increments; test concentrations of bentonite ranged from 0 (control) to 100g/liter, in about 20g/liter increments.


Test procedures for ferrochrome lignosulfonate (soluble) were the same as test procedures for the mud supernatants. Production-grade ferrochrome lignosulfonate powder (supplied by IMCO) was dissolved in seawater. Test concentrations ranged from 0 (control) to 16.67 g/liter, in about l g/liter increments. During the tests, concentrations of ferrochrome lignosulfonate were nearly constant, although 0.5 % (vol/vol) dilution per day was caused by water condensing from the air. We determined that ferrochrome lignosulfonate was chemically stable in seawater for at least 24 days but fresh preparations were made for each assay.

Concentrations of ferrochrome iignosulfonate (FCL) in the mud supernatants were determined with UV spectrophotometry at 278 + 1 nm using a standard curve. The maximum percentage of the toxicity (either ECso or LCso) caused by FCL in the supernatant was calculated as: (toxicity of FCL in the supernatant)/(toxicity of FCL in seawater) x 100. Other compounds probably did not contribute substantially to the absorbance at 278 nm because we detected only 64 % of the FCL reported by IMCO for used Cook Inlet mud. (The remaining 36 % was probably adsorbed to the particulates and, therefore, sedimented out with the solids during centrifugation.)

Because the alkalinity of the muds ranged from mildly to highly alkaline, tolerance of coonstripe shrimp to alkaline pH's (pH 8.0-12-0, in about 0.3 increments) was determined in a separate test. The test solutions were made by adding 2.5M NaOH to seawater. Procedures in these tests were the same as procedures for tests with the supernatants.

Mathematical methods

Using logit analysis (Finney, 1952), we determined the LCso'S and ECso'S from each replicated series of toxicant concentrations. (LCso'S are concentrations that kill 50 % of the larvae; ECso'S are concentrations that cause 50 % of the larvae to cease swimming. Note that mortality is a subset of the cessation of swimming category.) Concentrations were converted to log concentrations by the program used for the logit analysis. Correction for control mortality (Abbott, 1925) was applied as necessary. Mortalities of the controls were 2 + 1% at 96h. Reported LCso'S and ECso'S are averages of the replicate tests.

The LCso'S and ECso'S were analyzed with a single classification ANOVA (Sokal & Rohlf, 1969) and Scheff6's (1953) test at the 95 ~o

Toxicities of" mud components to larval shrimp and crabs 51

confidence level. Toxicity differences between muds were tested for king crab and coonstripe shrimp separately. Similarly, tolerance differences between species, as observed by LCso'S and ECso'S, were independently tested with the Scheff6 test.

The toxicity of particulates in the mud was determined indirectly from the toxicities of the suspensions and supernatants. We used an additive model as a first approximation because Sprague & Logan (1979) found that the joint action of drilling mud components ranged from additive to less than additive; roughly half their mixtures were generally in statistical agreement with an additive model. The total toxicity (ET), assuming an additive model, is ~ T = T~ + Tp, where the toxicity of the supernatant (Ts) is inversely proportional to the LCs0 of the supernatant, particulate toxicity (Tp) is inversely proportional to the LCso of the particulates and ET is inversely proportional to the LCso of the suspension. The fraction of the toxicity due to the particulates was, therefore, Tp/ET= 1 -(Ts/Y_.T)= 1 -[(suspended LCso)/(supernatant LCso)].

RESULTS

Rates of effect

The larvae responded more slowly to the muds than anticipated (Fig. 1). In tests with the most toxic mud, used Cook Inlet mud, inhibition of swimming began within 5 h for dock shrimp larvae (the most sensitive species) and within 24 h for king crab larvae (the most tolerant species). By 96 h the ECso curves for cessation of swimming and the LCso curves for mortality were relatively horizontal, indicating that the tests were long enough for the larvae to respond. Also, by 96h the ECso and LCso concentrations for a given species were beginning to converge.

Species sensitivities to used Cook Inlet mud

In general, shrimp larvae were significantly less tolerant (had lower LCso'S or ECso'S ) than crab larvae (Table 2). Dock shrimp were the least tolerant species; king crab and Tanner crab were the most tolerant species: (king crab -.. Dungeness crab ~ Tanner crab) > (coonstripe shrimp ~ kelp shrimp ~ dock shrimp), where > indicates a significant difference and ~ non-significant differences as determined by Scheff6's test at the 95 ~o

confidence level.


24

N

o

\~... ^

25 M 75 I M 125 168 HOUR8

Fig. 1. Maximum and minimum rates of effect of used Cook Inlet mud observed in the study, as measured by cessation of swimming (ECso'S: dashed lines) and mortality (LCso'S: solid lines). A. Dock shrimp larvae when exposed to suspensions of used Cook Inlet mud. B. King crab larvae when exposed to supernatants of used Cook Inlet mud. Vertical bars indicate the 95 ~o confidence intervals of the sample means. Curves were

fitted by least squares regressions.

Toxicities of the different muds to larvae of coonstripe shrimp and king crab

The range of toxicities to the larvae of coonstripe shrimp and king crab varied considerably among the six muds tested: LCs0's of the supernatants for all six muds ranged from I-6 to 82.4 ~o (vol/vol), and ECso'S ranged from 0.9 to 35.9 ~o (vol/vol) (Table 3). Generally, used Cook Inlet mud was the most toxic and used Homer mud was the least toxic. The relative toxicities of the muds to the larvae of coonstripe shrimp and king crab, from the most to the least toxic, were used Cook Inlet mud -~ new Prudhoe Bay mud > new Homer mud ,,~ Homer 'spud' mud _> used Prudhoe Bay mud > used Homer mud (> indicates significance in some, but not all, tests). Only the supernatant toxicities of the six different drilling muds were compared because, often, in tests with mud suspensions some of the larvae were buried during the initial few hours of settling, therefore biasing the results.

Toxic±ties of mud components to larval shrimp and crabs 53

TABLE 2 Tolerance of Shrimp and Crab Larvae to Suspensions and Supernatants of Used Cook Inlet Mud. Concentrations are Those that Caused Death (LCso'S) or Cessation of Swimming (ECso'S) of 50 ~o of the Animals in 96 h. ~ = Mean, CI = 95 % Confidence

Interval, - - = Not determined

Species 96-h LCso 96-h ECso

Suspension Supernatant Suspension Supernatant ( % vol/vol) ( ~ vol/vol) ( % vol/vol) ( % vol/vol)

£ ± CI ~ ± CI £ ± CI ~ ± CI

King crab 0.49 ± 0.001 11.9 ± 2.20 0.43 ± 0.03 4.31 + 0-34 Tanner crab 2.74 ± 1-44 4-69 ± 2.91 < 0.50 - - 1-46 ± 0-90 Dungeness crab 0.73±0.15 5.47+ 1.35 0.25±0-33 0.76±0-55 Coonstripe shrimp 0.28 + 0.00 1 "63 ± 0.77 < 0.40 0.88 ± 0.56 Kelp shrimp 1.19 ± 0-41 0.91 + 0.76 < 0.5 - - 0.45 + 0.19 Dock shrimp 0.06 ± 0-01 0.58 ± 0-28 0.05 ± 0.01 0.23 ± 0.07

TABLE 3 Toxic±ties of Six Drilling Mud Supernatants to Larvae of King Crab and Coonstripe Shrimp. Toxic concentrations are Those that Caused 50 ~o of the Larvae to Die (LCso'S) or Cease Swimming (ECso's) in 96 h. See Table 1 for a Description of the Muds. £ = mean,

CI = 95 % Confidence Interval, - = Not Determined

Drilling mud King Crab Coonstripe Shrimp

LC~o EC5o LCso ECso £ + C! .~ + CI £ ± CI £ ± CI

Prudhoe Bay (new) 8.04 ± 4.62 2.39 ± 6.87 4.23 ± 3.33 3.02 ± 2-44 Prudhoe Bay (used) - - - - 25.18 + 28.46 26.66 ± 99.97 Cook Inlet (used) 11.90 ± 2-20 4.31 ± 0.34 1.63 ± 0.77 0.88 ± 0-56 Homer(new) 82.40±59-83 22.23+ 10.63 15.92+ 1.59 8.43±4.11 Homer (used 'spud') 51.01 ± 99.37 21.78 ± 3.21 22.08 ± 2.62 13.81 ± 0.98 Homer(used) 70.84±7.16 33.14±9.12 38.36±0.14 35.94_+6-70


Toxicity of the mud fractions

The suspensions of used Cook Inlet mud were more toxic to all six species than the supernatants (Table 2). The LCso'S for suspensions ranged from 0.06 to 2.7 % (vol/vol); LCso'S for supernatants ranged from 0-9 % to 11.9% (vol/vol). The ECso'S ranged from 0.05 to 0.4% (vol/vol) for suspensions and from 0.2 to 4.3 % (vol/vol) for supernatants.

Most of the toxicity of used Cook Inlet mud was due to particulate matter, rather than soluble chemicals. Based on an additive model, particulate matter in used Cook Inlet mud accounted for a mean of 88 + 6 % of the LCso'S of all six species (Table 4). Toxicity of particulate matter determined from the ECso'S (78 + 11%) was similar to the toxicity determined from the LCso'S.

Toxicity of specific components

Barite and bentonite were the primary components (91 ~o by weight) of the particulate fraction in used Cook Inlet mud (Table 5), but they were not particularly toxic when tested alone: concentrations > 16 g/liter were required to stop the larvae from swimming. At these concentrations,

TABLE 4 Contribution of Particulates to the Total Toxicity of Used Cook Inlet Mud. The Per Cent of the Total Toxicity Due to Particulates (I - [(LCso or ECso of Suspension)/(LCso or

ECso of the Supernatant)]) as Calculated from Either LCso'S or ECso'S

Species Per cent total toxicity due to particulates as measured by: LCso tests ECso tests

(%) ~%)

King crab 95.9 90.0 Tanner crab 80.8 - - Dungeness crab 86.6 67.3 Coonstripe shrimp 82.8 - - Kelp shrimp a 93-6 - - Dock shrimp 90.5 78.2

Mean 88-4 78-3 SD 6.0 11.5 +CI +6.3 +28-6

a 144 h data used to allow for variations in response rates.

Toxicities of mud components to larval shrimp and erabs 55

TABLE 5 Composition of Used Cook Inlet Mud a

Use Composition Quantity (g/liter)

Increase density Barite (BaSO4) 570.0 Viscosifier Bentonite (Montmorillinite clay) 42,8 Dispersant Ferrochrome lignosulfonate 28.5 Emulsifier Suifonated asphaltene 17.1 Viscosifier Polyanionic cellulosic polymer 5.7 Filtration control Ferrochrome lignosulfonate, lignite, sodium car-

bonate, sodium nitrilotriacetic acid 2-9 Thinner Suifonated quebracho containing tannins of the

condensed type 2.9 pH control Sodium hydroxide (NaOH) 2.9

Water ( % vol/vol) 75 % Solids ( % vol/voi) 25 %

a Data supplied by 1MCO Services, the supplier of used Cook Inlet mud.

suspensions of barite and bentonite form an optically dense medium which probably causes swimming difficulties. The concentrations of barite and bentonite in toxic suspensions of used Cook Inlet mud were 2 ~o (mean) or less of the toxic concentrations of barite or bentonite alone (Table 6). We believe this large difference is due to the differences in particulate behavior between the tests (i.e. particulates remained suspended in used Cook Inlet mud suspensions, but settled out quickly in all other mud tests and in the pure barite and bentonite tests). Burial of the larvae precluded measurements of LCso'S for either barite or bentonite.

Ferrochrome lignosulfonate probably accounted for about half (37 to 70 ~,) of the toxicity of the supernatant of used Cook Inlet mud (Table 6). Since the supernatant accounted for only 12 + 6 ~o of the total toxicity of used Cook Inlet mud, ferrochrome lignosulfonate therefore only contributed 4 to 8 ~o to the total mud toxicity. The response rates of larvae of dock shrimp and Dungeness crabs exposed to ferrochrome lignosulfonate were similar to the response rates of these larvae exposed to the supernatant of used Cook Inlet mud, which also implies that ferrochrome lignosulfonate is a major contributor to supernatant toxicity.

Alkaline conditions caused deaths only in the most alkaline mud, new


T A B L E 6

Percentage of the Total Toxicity of Used Cook Inlet Mud due to Barite, Bentonite, or Ferrochrome Lignosulfonate. The Per Cent Total Toxicity due to Pure Compounds = (Concentration of Pure Compound at the ECso or LCso of the Pure Compound)/(Concentration of Pure Compound in Used Cook Inlet Mud at the ECsolor LCso of the Suspension or Supernatant of Used Cook Inlet Mud) x 100. All LCso'S or

ECso'S Were Determined at 96 h

Compound LCso or ECso LC~o or ECso Per cent effect concentration concentration of due to pure of compound compound in compound in used

in mud solution pure compound Cook Inlet mud tests (%)

(g/liter) (g/liter) Supernatant Suspension

Barite a

Dungeness crab ECso 1.42 71.4 + 27 1.99 Dock shrimp ECso 0.28 16.2 + 24 1.73

Mean 1.86

Bentonite a Dungeness crab ECso 0.11 81.6 +_ 50 0.13 Dock shrimp ECso 0.02 24.8 + 10 0-08

Mean 0.11

Ferrochrome lignosulfonate h Dungeness crab LCso 1-00 1-44 + 0.53 69.5 x 0.12 c = 8.34

ECso 0.14 0.33 + 0.20 42.4 5.09 Dock shrimp LCso 0-11 0.29 + 0.18 36.9 4.43

ECso 0.04 0.11 + 0.03 40.0 4.80

Mean 47.2 5.66

Based on the 96-h suspension tests. b Based on the 96-h supernatant tests. c Obtained from Table 4 ( 1 0 0 % - 88%--- 12%).

Prudhoe Bay mud (Fig. 2). The 96-h pH LCso for coonstripe shrimp larvae was 9.7--10.4. This pH was exceeded by the supernatant of new Prudhoe Bay mud at a concentration of 3-8 ~o (vol/vol). Because the LCso of new Prudhoe Bay mud was 0.9 to 7.6 ~o (vol/vol), the toxicity of this mud could be attributed primarily to its alkalinity. In contrast, the pH of used Cook Inlet mud at the LCso for coonstripe shrimp larvae was about

Toxicities o f mud components to larval shrimp and crabs 57

"r G.

! I ID 211 3 B 4 E - - 8 E PERCENT HUO $UPERNATANT

Fig. 2. Contribution of pH to the toxicity of drilling muds for coonstripe shrimp. The shaded area indicates the zone of lethal alkalinity for coonstripe shrimp. Alkalinity is the major contributor to toxicity only when pH of a supernatant at the LCso overlaps the

shaded area (new Prudhoe Bay mud only).

8.0, well below the lower confidence interval of the pH LCs0 for coonstripe shrimp.

DISCUSSION

The toxicities of the drilling muds that we tested varied widely (0.58 to 82"4~o for the supernatants) probably because of differences in the original components and their proportion in the mud, the age of the mud, its history of use, the depth of drilling and the formations penetrated. The LCso'S and ECs0's in our study, however, were similar to the LCso'S and ECso'S in other studies that tested the sensitivity of crustacean larvae to drilling muds. For example, Gerber et al. (1980) observed a 96-h supernatant LCso of 1.7 ~o for Stage I pink shrimp (Pandalus borealis) larvae and 0-5 ~o for Stage V American lobster (Homarus americanus) larvae. Carr et al. (1980) observed a 96-h supernatant LCso of 2.7 ~o in l- day-old Mysidopsis almyra juveniles. Neff et al. (1980) observed a 96-h supernatant LCso range of 1.17-2.57 ~0 for Stage I grass shrimp

58 Mark G. Car&, Stanley D. Rice

(Palaemonetes pugio) larvae. All of these values fall within the 96-h toxicity range observed in our study.

Used Cook Inlet mud was more toxic than the other muds for two reasons: first, the particulates in used Cook Inlet mud remained in suspension and subjected the larvae to continual physical contact with suspended particles. In contrast, in tests with barite and bentonite, particulate matter settled quickly, so the larvae were only briefly exposed to physical contact with suspended particles. We believe this is why the calculated per cent effect due to barite and bentonite (Table 6) is so much lower than expected. Secondly, used Cook Inlet mud contained a much higher concentration of ferrochrome lignosulfonate, and possibly other additives, than the other muds.

The second most toxic mud, new Prudhoe Bay mud, was toxic for entirely different reasons: it contained so much sodium hydroxide that the undiluted supernatant had a pH of 13-2. The diluted supernatant produced lethal alkalinities at concentrations as low as 3"8 ~o (vol/vol) for coonstripe shrimp larvae. In marine waters highly alkaline muds should not pose significant toxicity problems to marine plankters because of rapid dilution and the excellent buffering capacity of seawater.

Mud suspensions were, on average, over seven times more toxic than supernatants. Based on an additive model, particulates (primarily barite and bentonite) caused 88 + 6 ~o of the total toxicity of used Cook Inlet mud. We feel an additive model best approximates the relationship between the two mud phases (solid and soluble) because we obtained the supernatant from the mud after any possible solid-soluble interactions. We also note other studies have observed additive to less-than-additive (antagonistic) toxicities in tests with drilling fluid components (Sprague & Logan, 1979). We suspect the antagonistic reactions they observed were due to the adsorption of soluble compounds on the particulates. The contribution of particulates to the toxicity of the muds we tested varied considerably because of the physical differences between them. Land (1974) reported that barite and bentonite accounted for 45-60 ~ of the total theoretical toxicity of a drilling mud to rainbow trout (Salmo gairdneri).

Barite and bentonite had low toxicities, as indicated by the fact that they only affected survival for the first few hours of exposure. After the particles settle out of the water, no further effects were observed. Other researchers have also found that the toxicities of barite and bentonite are low (Daugherty, 1951 ; Logan et al., 1973; Sprague & Logan, 1979). We


suspect that if the test solutions were continually mixed during the assays in order to keep the particulates in suspension, the toxicities of these compounds would have been much greater. For example, the particulates in used Cook Inlet mud remained in suspension throughout the assays and the effect due to particulates was much higher (88 ~) than expected (1.86~ and 0.11 ~) from tests with individual compounds. The detrimental effects of these suspended compounds were probably mechanical: suspended solids may cause mortalities through abrasion, erosion, or the clogging of respiratory surfaces (Sprague & Logan, 1979; Gerber et al., 1980). The toxic effect of bentonite may also be related to its absorptive capacity as suspended solids are passed through the gut (Robinson, 1957).

In our studies, the supernatant of used Cook Inlet mud accounted for only 12 _+ 6 ~ of the total mud toxicity. The toxicity of the supernatant was probably a subacute effect due to dissolved chemicals, which may increase the rate at which larvae expended energy. In Carr et al.'s (1980) study, sublethal concentrations of mud supernatants increased food consumption, increased initial respiration rates, and decreased the growth of juvenile Mysidopsis almyra. In the study by Carr et al. (1980) respiration rates eventually returned to normal, but growth decreased further, probably because energy was shunted into homeostatis, and the amount of energy available for growth was reduced.

The slow reaction of the sensitive larvae tested in our study to drilling mud suspensions indicates that the response was due to physical, rather than chemical, factors. Crustacean larvae equilibrate quickly with compounds present in the water surrounding them (C. C. Brodersen, personal communication concerning radioisotope uptake studies with coonstripe shrimp larvae). If the toxic action of the drilling muds had been primarily chemical the effects would have been much more rapid. For example, when the larvae of king crab, Dungeness crab, coonstripe shrimp and kelp shrimp collected from the same group of larvae used for this study were exposed to acutely toxic substances they ceased swimming within minutes (Rice et al., 1981). In contrast, these larvae responded much more slowly to drilling mud suspensions and supernatants.

The two potential sources of pollutants from the oil wells, petroleum hydrocarbons and drilling muds, are very different chemically and toxicologically. The water-soluble fractions of crude oil are very toxic chemically and often cause crustacean larvae to cease swimming within minutes after exposure (Rice et al., 1981). In contrast, crustacean larvae


respond slowly to high concentrations of drilling muds (up to several days for swimming cessation) and the toxicity is principally physical. When exposed to the water-soluble fraction of crude oil, the 96-h ECso'S for larvae of king crab, coonstripe shrimp, Dungeness crab and kelp shrimp ranged from 0.4 to 2-5 ppm (Rice et al., 1981)--one thousandth to one ten-thousandth of the ECs0's for the supernatants of the muds that we tested. In our tests, ferrochrome lignosulfonate was more toxic than the mud supernatant, yet still only one ~hundredth as toxic as the water- soluble fraction of crude oil. This indicates the dissolved chemicals in the drilling muds we tested are not present at concentrations which are acutely toxic.

Drilling muds contaminated with petroleum hydrocarbons from oil- bearing formations could be more toxic than uncontaminated muds. However, small quantities of hydrocarbons would probably be adsorbed on the particulate matter and, therefore, might not be as toxic as a comparable concentration of hydrocarbons in the water-soluble fraction of a crude oil. We did not analyze the hydrocarbons in our muds, but neither oily slicks nor petroleum odors were detected. (We did try to extract petroleum hydrocarbons from a mud with dichloromethane, but large quantities of the solvent were trapped in the solids and could not be recovered by centrifugation or filtration.)

Under most conditions in the marine environment, drilling mud probably would not measurably affect planktonic crustacean larvae because the muds are not acutely toxic and because the larvae are only briefly exposed to toxic concentrations of drilling mud. In most situations, muds discharged into the ocean are rapidly diluted. Such dilution would generally reduce toxic concentrations to harmless levels within a few feet of the discharge point. For example, dilutions from 500:1 (water: mud) to nearly 1000:1 were found within 3 m of the discharge pipe from a drilling platform (Ray & Meek, 1980). Within 300m of the discharge pipe, total suspended solids from the muds were negligible (Ray & Meek, 1980). It is conceivable, however, that in small estuaries and areas with stable density gradients, drilling muds could reach toxic levels.

AC KNOW L E DGE MENT

We wish to thank Chris Brodersen for her expert larval culturing and handling advice, and Dione Cuadra for her technical work. Funding


support was provided by the US Depar tment of Interior's Bureau of Land Management through the Outer Continental Shelf Energy Assessment Program of the National Oceanic and Atmospheric Administration.

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Documents

Toxic contributions of specific drilling mud components to larval shrimp and crabs