LONG-TERM CHANGES IN SEDIMENT BARIUM INVENTORIES ASSOCIATED WITH DRILLING-RELATED DISCHARGES IN THE SANTA-MARIA BASIN, CALIFORNIA, USA

Nine-year (1986-1995) records of barium (Ba) concentrations in surfici al, subsurface, and suspended sediments near offshore oil and gas plat forms in the Santa Maria Basin, California, USA, were analyzed to eval uate temporal trends related to drilling activities. These trends prov ide important information on the long-term effects of drilling dischar ges on geochemical conditions. Drilling during the 1986 through 1989 ( phase II) monitoring period resulted in significant changes in Ba conc entrations in suspended particles and surficial sediments, whereas the relatively shorter 1993 through 1994 (phase III) drilling operations resulted in only minor increases in Ba concentrations in suspended sed iments. Residual excess Ba was present in some sediments within 500 m of the platforms at concentrations up to an order of magnitude above b ackground. These elevated levels probably were associated with cutting s particles deposited near the base of the platforms. Calculated exces s Ba in sediments within 500 m of the platforms represented 6 to 11% o f the total Ba discharged during the two drilling periods.