Offshore oceanographic and environmental monitoring services for the strategic petroleum reserve: annual report for the Bryan Mound site from September 1983 through August 1984.
Hann, R.W., Jr., C.P. Giammona, and R.E. Randall
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The Department of Energy's Strategic Petroleum Reserve Program has been discharging brine into the coastal waters offshore of Freeport, Texas since March 10, 1980. This report describes the findings of a team of Texas A & M University scientists and engineers who have conducted a study to evaluate the effects of the Bryan Mound brine discharge on the marine environment during the period from September 1983 through August 1984. The study addresses the areas of physical oceanography, brine plume, water and sediment quality, nekton, bemthos, and data management. The ambient physical environment and its temporal and spatial variability were studied by means of continuously recording in situ current/conductivity/temperature meters and twelve, one-day synoptic hydrographic cruises. Mean currents were downcoast at all depths during September 1983 through May 1984 and upcoast or near zero during June through August 1984. Alongshelf currents and vertical velocity shear were very strong during January and May. The monthly mean alongshelf components of currents followed closely that of wind in terms of direction and strength. An unusually cold winter caused mean water temperatures to drop very rapidly to an early minimum in January. Mean vertical temperature stratification existed from April through August, and was quite strong in July because of upwelled cooler bottom water. Streamflow from the Mississipi and Atchafalaya rivers was high during December and March through May. The maximum occurred in May 1984. The quarterly water and sediment quality data showed that a small increase in the salinity and the sodium and chloride ions occurs in the bottom waters and sediment pore waters near the diffuser relative to those valukes measured at staitons farther away. Brine pond and Brazos river intake water showed increases in lead, zinc, iron, and cadmium in the brine relative to the leach water, but the levels in the vicinity of the diffuser were similar to the control stations and never exceeded the maximum allowed EPA levels for marine life. Analysis of the soluble metals in the sediment pore waters indicated that nickel and chromium exceeded EPA levels fo aquatic organisms during the summer. Hydrocarbons in the river and offshore were of natural origin whereas those in the brine were of petroleum origin. Results of a special , low molecular weight hydrocarbon sample collected at the brine pond in August 1984 show the concentraiton of these hydrocarbons were an order of magnitude higher than the heavy molecular weight hydrocarbons. Data from the brine plume study show the maximum areal extent within the +1 o/oo above ambient salinity contour was 45.0 km squared and the highest measured salinity was 39.7 o/oo. The longest distance to the +1 o/oo above ambient salinity contour was 7.5 km. The highest measured vertical extent of brine plume was 4 .0 km. The highest measured above ambient salinity contour (+5 o/oo) was measured in February 1984. Predictions of above ambient salinity showed that the highest above ambient salinity value was +6.4 o/oo and that the +5 o/oo contour opccured more frequently than in the previous reporting period. This was attributed to slighlty lower brine discharge rates. The mean predicted areal extent for this period was 17.7. 7.5, 4.4, and 3.4 km squared for the +1 through +4 o/oo above ambient salinity contours. THe nekton investigations showed that the brine disposal at Bryan Mound has resulted in no profound disruption of th overall nekton community inhabiting the diffuser area. The data since the inception of brine disposal in March 1980 show: 1) no dramatic lethal effects on nekton have been observed at any station during any cruise; 2) responsses of nekton abundance, biomass and diversity to brine have been inconsistent; and 3) within-cruise indivdual degree of freedom contrasts have been generally non- significant and brine effects a minor source of the observed model variation. Reasons for the absence of brine impacts seem correlated with several factors, among them: the intrinsic dynamism of shrimp and fish populaitons, the negligible area covered by the brine plume in relation to areas over which stocks may range, and maximum brine elevations that are below levels known to cause mortality or evoke avoidance responses in laboratory tests. The benthic quarterly data from 26 stations, including 11 collections made after the diffuser outflow rate was increased to 1,000,000 barrels/day, show the total numbers of species were depressed in an arc around the diffuser, but not at the diffuser station. In contrast, the total abundances at the diffuser station were depressed, and a zone of enhancement occurred in an arc around the diffuser. Cluster analysis and principal components analysis did not indicate any brine effect. Effects of brine discharge on benthic organisms over increasing distances indicated that in February 1984 the near-diffuser stations had a greater number of individuals than the outer ring stations. In May the highest populaiotns were noted at the 1000-meter ring staitons. No significant differences in population levels were noted during November and August.