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Abstract-This study attempts to predict biological toxicity and benthic community impact in sediments collected from two southern California sites. Contaminant concentrations and grain size were evaluated as predictors using a two-step multivariate approach. The first step used principal component analysis (PCA) to describe contamination type and magnitude present at each site. Four dominant PC vectors, explaining 88% of the total variance, each corresponded to a unique physical and/or chemical signature. The four PC vectors, in decreasing order of importance, were: (1) high molecular weight polynuclear aromatic hydrocarbons (PAH), most likely from combusted or weathered petroleum; (2) low molecular weight alkylated PAH, primarily from weathered fuel product; (3) low molecular weight nonalkylated PAH, indicating a fresh petroleum-related origin; and (4) fine-grained sediments and metals. The second step used stepwise regression analysis to predict individual biological effects (dependent) variables using the four PC vectors as independent variables. Results showed that sediment grain size alone was the best predictor of amphipod mortality. Contaminant vectors showed discrete depositional areas independent of grain size. Neither contaminant concentrations nor PCA vectors were good predictors of biological effects, most likely due to the low concentrations in sediments.

Pre- and post-drilling environmental studies were conducted around an exploration well drilled in the south Caspian Sea in a water depth of 145 m using both water-based and synthetic-based drilling muds. Eight months after completion of the well, chemical analyses of surface sediment samples for linear alpha olefins, polycyclic aromatic hydrocarbons, saturated hydrocarbons, and selected metals confirmed that the spatial extent of the settled cuttings and mud was limited to an approximate 400-m radius. Cross-sectional photographic images of the seafloor showed significant oxygen demand and presence of sulfidic sediments within 50 m of the well. Coincident benthic samples were examined for total infaunal abundance, species richness, biomass, and distribution of major taxa. The macrofaunal groups Amphipoda, Cumacea and Gastropoda were present in pre-drilling and post-drilling reference samples, but were not present in post-drilling samples collected within 50 m of the well. However, these groups had enhanced abundances, likely in response to moderate organic enrichment at 200 and 400 m distances, where there was no evidence of excessive oxygen demand. Ostracod crustaceans were rare in pre-drilling samples, but were the most abundant post-drilling group in all sediments collected within 400 m of the well site, indicating that elevated organic matter may be a strong macrofaunal attractor. Overall results indicate that changes in sediment chemical concentrations and benthic community composition associated with drilling did not extend significantly beyond 400 m from the well site, and that recolonization of the site was underway, eight months after completion of the well. Introduction A range of studies on environmental impacts associated with drilling and discharge of drilling muds and cuttings have been conducted in aquatic environments over the past 30 years1–7. Many of these studies focus on impacts to the benthic macrofauna, which are recognized as effective sentinels or indicator organisms for the monitoring of point source discharges and temporal impacts due to their relative immobility, typically short life spans, and sensitivity to physical-chemical features of sediments8–13. Petroleum exploration can impact benthic macrofauna as a result of discharge and seafloor accumulation of drilling cuttings and adhered muds, and release of hydrocarbons2–4,14. Documented long term effects from the discharge of drilling muds and cuttings include smothering, organic enrichment and toxicity from heavy metals and hydrocarbons. Use of diesel-containing oil-based muds in the North Sea often resulted in toxic concentrations of petroleum hydrocarbons in near-platform sediments15,16. Conversion to synthetic-based, low-toxicity drilling fluids has been shown to reduce these impacts, although physical impacts such as smothering and habitat alteration remain17,18. Recently, exploration and production activities have increased in inland seas, which are less saline, and therefore differ in species composition from offshore continental shelf environments. Impacts from oil and gas exploration in many of these seas are not well documented; even so, they should not be assumed to mimic those in offshore marine ecosystems. Lacking knowledge of the effects of low-toxicity synthetic muds in inland sea environments, a comprehensive set of environmental studies, including physical oceanography as well as chemical and biological assessments, was undertaken at several exploration well sites in the south Caspian Sea. This paper reports some of these studies, and examines chemical and biological results for pre- and post-drilling sediments collected at and around a single exploration well site located in the Azerbaijan sector of the south Caspian Sea.

This study attempts to predict biological toxicity and benthic community impact in sediments collected from two southern California sites. Contaminant concentrations and grain size were evaluated as predictors using a two‐step multivariate approach. The first step used principal component analysis (PCA) to describe contamination type and magnitude present at each site. Four dominant PC vectors, explaining 88% of the total variance, each corresponded to a unique physical and/or chemical signature. The four PC vectors, in decreasing order of importance, were: (1) high molecular weight polynuclear aromatic hydrocarbons (PAH), most likely from combusted or weathered petroleum; (2) low molecular weight alkylated PAH, primarily from weathered fuel product; (3) low molecular weight nonalkylated PAH, indicating a fresh petroleum‐related origin; and (4) fine‐grained sediments and metals. The second step used stepwise regression analysis to predict individual biological effects (dependent) variables using the four PC vectors as independent variables. Results showed that sediment grain size alone was the best predictor of amphipod mortality. Contaminant vectors showed discrete depositional areas independent of grain size. Neither contaminant concentrations nor PCA vectors were good predictors of biological effects, most likely due to the low concentrations in sediments.

The effects of linear alpha olefin (LAO) nonaqueous drilling fluid on benthic macrofauna were assessed over a six year period at a southern Caspian Sea petroleum exploration site. A wide-ranging, pre-drilling survey identified a relatively diverse shelf-depth macrofauna numerically dominated by amphipods, cumaceans, and gastropods that transitioned to a less diverse assemblage dominated by hypoxia-tolerant annelid worms and motile ostracods with increasing depth. After drilling, a similar transition in macrofauna assemblage was observed with increasing concentration of LAO proximate to the shelf-depth well site. Post-drilling results were consistent with a hypothesis of hypoxia from microbial degradation of LAO, supported by the presence of bacterial mats and lack of oxygen penetration in surface sediment. Chemical and biological recoveries at ≥200m distance from the well site were evident 33months after drilling ceased. Our findings show the importance of monitoring recovery over time and understanding macrofauna community structure prior to drilling.