Effects of two contrasting agricultural land uses on shallow groundwater quality in the San Joaquin Valley, California: design and preliminary interpretation

Neil M. Dubrovsky, Karen Burow, and Jo Ann M. Gronberg

In:
Wagner, B.J., Illangasekare, T.H., and Jensen, K.H., eds.1995, Models for assessing and monitoring groundwater quality: Wallingford, Oxfordshire, U.K., International Association of Hydrological Sciences, publication 227, proceedings, p. 49-58.

Abstract:
From 1992 through 1994, the San Joaquin-Tulare Basins Study team of the USGS National Water Quality Assessment program investigated the occurence and distribution of water quality constituests in shallow groundwater underlying two areas of different agricultural land uses: almond orchards and vineyards. The study was restricted to the alluvial fans of the eastern San Joaquin Valley, the area of most groundwater use in the valley. A geographic information system (GIS) was used to delineate the distributions of the two target land uses, to evaluate ancillary data, and to select candidate wells that fit prescribed criteria. Twenty domestic water supply wells were sampled in each of the two areas. In addition, pairs of observation wells were installed and sampled at five of the sites in each area to evaulate whether the water quality in the domestic wells reflects that of the shallow groundwater underlying the target land use. A preliminary evaluation of the results shows that nitrate concentrations in the shallow groundwater are significantly higher in the in the almond orchard areas than in the vinyard area (p = 0.005). In contrast, concentrations of 1,2-dibromo-3-chloropropane (DPCB) were higher in the vineyard area than in the almond orchard area (p=0.032). The most frequently detected pesticides in groundwater underlying both areas were simazine, atrazine, and desethylatrazine (an atrazine degradation product). These observations are explained, in part, by differences in chemical application and hydrogeologic factors.