SU0012814 - Laserfiche WebLink

Home Browse Search

-Mr. Mike Huggins February 9, 2009 Groundwater Monitoring Plan Page 3 of 8 Granite Construction Company,Vernalis Plant Corcoran Clay. For the purposes of this report,wells completed above the Corcoran Clay are referred to as shallow and wells completed below the Corcoran Clay are referred to as deep(see Figure 2). Groundwater levels observed in the shallow wells have been generally stable since monitoring began in 1994,typically fluctuating seasonally three feet or less,with seasonal high water levels generally observed in late fall and winter,and the seasonal low water levels observed in late sp nd summer (Figure 3). Groundwater deptthsan e shal9aw.aquifer zone have ranged from abo t �et in the northwest portion of them itored ar a t°'�155 Met in the southeast, and the corresp�dlzjg groundwater elevations range from abo t 87 to 75 et,rrlsl/(above mean sea level) (Attachment 2). In the mid to late 1990's, seasonally hi ndwater levels rose approximately four to five feet. Since 1998, seasonally high water levels have gradually declined such thatgroundwater elevation differences across.. . the monitored area have decreased. Long-term fluct tions of show groundwater levels in the area are depicted by the hydrograph for Well 4S/6E-5A (Figure 4),which has been monitored semi-annually by the California Department of Water Resource si 960. Groutidwater levels in this well have fluctuated in a manner reflecting the/ historical hydrologic conditions: declining slightly during the drought of 1976-77,recovering to the highest observed levels during the wet period of 1983-86,and declining to the lowest observed levels during the drought of 1987-92. As observed in the other shallow network wells,water levels in Well 4816E-5A1 slightly recovered subsequent to the 1987-92 drought through 1998,and have since then slightly declined through to the present. Contours of equal groundwater elevations prepared with data from the shallow network wells show that the general direction of groundwater flow has ranged from southeasterly to east-northeasterly. For example,during times of seasonally high water levels;groundwater levels indicate east-southeasterly to easterly flow, including spring 1994 with comparatively high water levels(Figure 5)and the period of the highest historically recorded groundwater elevations in spring 1998 (Figure 6). The water level elevations in spring 2000 were relatively low compared-to groundwater conditions observed in previous spring seasons,however,the predominant direction of flow was also to the east-southeast(Figure 7). In spring 2002, overall groundwater elevations were very similar to the spring 2000 elevations,but the groundwater flow direction was east-northeasterly(Figure 8). Seasonal groundwater level fluctuations have been small in recent years and groundwater elevation differences across the monitored area have decreased..The contours of equal groundwater elevation prepared for May 2008 indicate groundwater flow in an easterly direction (Figure 9)." Groundwater level data obtained from the California Department of Water Resources database for shallow wells in the area of the Granite Vernalis Plant were used to generate contour maps of equal groundwater elevations for 1958 and 1970 (Attachment 3). In 1958, a time of relatively low water levels, the overall groundwater flow direction was to the southeast. In 1970,when groundwater elevations were about 18 feet higher below the Granite Plant area,the direction of flow was east- southeasterly to easterly. These maps also show that groundwater elevation differences across the monitored area decreased from 1958 to 1970 with rising water levels. Groundwater levels observed in the deep wells are typically lower,and exhibit greater seasonal and annual fluctuation than levels in the shallow wells(Figure 10, see also Attachment 2). Water levels in the deep aquifer zone have ranged in depth from about 190 to 340 feet,with up to 40 to 60 feet of seasonal fluctuation. In the early 2000s, seasonally high groundwater levels in deep zone wells South Tracy#1, Brown's#1, and Brown's New Well declined by up to about 60 feet. In the early 2000s, substantial recovery occurred. Seasonally high water levels since 2005 have been lower than during the 1990's but appear stable. These lower groundwater levels are likely due to an increased demand (pumpage) on the deep aquifer zone from local agriculhiral interests and industrial operators. For example, pumpage from Teichert's production wells has increased with increased operations at the Vernalis plant. In addition,a number of deep wells were reported to have been installed in the area t © LUHOORFF 6 SCAI_MANINI .C D N S U L T I N G E N G I N E E R S - -. �' :