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Groundwater contamination at the Kesterson Reservoir, California: 1. Hydrogeologic setting and conservative solute transport
Author(s) -
Benson Sally M.,
White Art F.,
Halfman Susan,
Flexser Steven,
Alavi Moshen
Publication year - 1991
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/91wr00265
Subject(s) - aquifer , hydrogeology , groundwater , biogeochemical cycle , hydrology (agriculture) , plume , geology , infiltration (hvac) , nitrate , san joaquin , environmental science , groundwater flow , environmental chemistry , soil science , chemistry , geotechnical engineering , thermodynamics , physics , organic chemistry
Shallow groundwater under Kesterson Reservoir, Merced County, California, was contaminated over a period of 6 years while the reservoir was used for disposal of brackish subsurface agricultural drainage water from farmlands in the San Joaquin Valley. Drainage water contained an average of 10,000 mg/L total dissolved solids (TDS), high concentrations of selenium (300 μg/L), nitrate (37 mg/L as nitrogen), boron (15 mg/L), and lesser amounts of mercury, chromium, and molybdenum. Infiltration and lateral migration of this water created a 20‐m‐deep plume of high‐TDS, boron‐rich water covering an area of about 5 km 2 . Transport of selenium and nitrate was inhibited by biogeochemical reactions in the pond bottom sediments, so that migration of these two constituents into the aquifer was limited. This paper focuses on physical parameters affecting seepage and lateral migration of nonreactive solutes and on hydrogeological factors associated with selenium break‐through into the shallow aquifer. An accompanying paper describes biogeochemical processes that limit nitrate and selenium migration (White et al., this issue). The regional hydrogeologic system at Kesterson Reservoir is dominated by the presence of four lithostratigraphic units. These are, in order of increasing depth, a fine‐grained surfical layer extending to depths of as much as 8 m, a semiconfined sandy aquifer extending to 60 m, a widespread lacustrine clay confining layer, and a confined sandy aquifer. Within each of these units, order of magnitude variations in the hydrologic properties of the sediments occur ubiquitously over lateral distances of tens of meters and vertical distances of a fraction of a meter. Seepage rates from the ponds into the underlying aquifer are dominated by local hydrogeologic properties and thickness of the fine‐grained surface layer. Selenium migrated into the aquifer in an area where the fine‐grained surface layer was thin or absent. Once in the aquifer, local rates of migration are controlled by the small‐scale structure and hydrogeologic properties of the sediments. The aquifer is composed of a heterogeneous mixture of sands, silts, and clays. Individual sand beds, which average about 0.3 m thick, do not appear to extend over lateral distances of more than 30 m. The large range of velocities combined with a limited lateral extent of individual layers are expected to create a broadly dispersed plume of solutes migrating downgradient from Kesterson. A noninvasive electromagnetic survey of electrical conductivity distributions in the shallow aquifer appears to confirm this conceptual model for transport in the shallow aquifer.