Pore-Water Geochemistry and Hydrostratigraphy at Two Continuously Cored Sites in Long Beach, CA

M. T. Land¹, R. J. Rosenbauer², T. D. Lorenson², and J. J. Montrella^1
1U.S. Geol Survey, San Diego, CA
²U.S. Geol Survey, Menlo Park, CA

Intensive pumping of ground water throughout the coastal plain of Los Angeles during the early half of the 20th century has dramatically altered the ground-water flow system. Along the coast, declining water levels led to widespread seawater intrusion and deterioration of fresh ground-water resources. To assess current ground-water conditions, controls on seawater movement, and geochemistry of the sedimentary deposits, two 1,400-foot continuously cored boreholes were drilled in Long Beach, California by the USGS, in cooperation with the Water Replenishment District of Southern California and the Los Angles County Department of Public Works. Pore water squeezed on-site from more than 440 targeted sediment samples was analyzed for total salinity (%NaCl equivalents), pH, major ions, TIC, stable isotopes, and organic acids. Core material was analyzed for water content, extractable organics, major elements, and TIC/TOC. Pore water is relatively fresher in coarse-grained deposits at theLBPC site (adjacent to San Pedro Bay), and in the fine-grained deposits at the LWEB site (~5 km inland) owing to changes in the inland freshwater head pressures. The pH is inversely related to salinity and likely controlled by carbonate equilibria. Water content ranges from 17 to 45%, and generally decreases with increasing depth for fine-grained deposits at both sites. At  LBPC, pore-water salinity decreases smoothly with increasing depth from a near seawater value of 3.2% to 0.11%. Pore-water salinity at LWEB is typically less than 0.10%, although in a section of the Gage aquifer (84 m) it increased to 0.46%. High chloride (2,900 mg/L) at this interval could result from mixing of 15% seawater with fresh ground water. However, preliminary Cl/Br ratios and major-ion data indicate that elevated salinity in other intervals is influenced by a seawater and (or) oil-field brine source. Deuterium and 18O were measured to determine the source and relative age of ground-water recharge to the deposits underlying each site. A statistical analysis of fluid compositions was used to correlate stratigraphic units between sites.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California