By Randall T. Hanson
U.S. GEOLOGICAL SURVEY
Water Resources Investigations Report 03-4096
Sacramento, California 2003
Pajaro Valley is a coastal watershed of 160 square miles located along Monterey Bay north of Elkhorn Slough and south of the city of Santa Cruz. The valley has been predominantly developed for agriculture since the late 1800s. In 1984 the Pajaro Valley Water Management Agency (PVWMA) was formed and was delegated with the responsibility of the management of the water resources within the Pajaro Valley by the State of California. About 84 percent of the water is used for agriculture and 16 percent is used for industrial and municipal water supply; almost all of the demand is supplied by ground water. Ground-water pumpage varies with seasonal and climatic periods.
The alluvial aquifers are composed of Quaternary- and Tertiary-aged sediments that are layered marine and terrestrial coarse-grained deposits separated by extensive fine-grained deposits that potentially restrict vertical movement of ground water and seawater intrusion in the coastal subareas. The coarse-grained deposits, which persist over large areas, control pumpage and related seawater intrusion. The Aromas Sand crops out throughout the north and central parts of the PVWMA area and offshore on the continental shelf and in Monterey submarine canyon. Because many of the wells in the coastal and inland subregions are screened at depths of 200 to 400 feet below land surface, a direct avenue is provided for seawater intrusion through the coarse-grained deposits of the shallower alluvium and Aromas Sand. Geophysical logs from monitoring wells indicate discrete zones of saline water that are related to pumpage and seawater intrusion in the aquifers of the shallower alluvium and upper Aromas Sand in the upper-aquifer system and to deeper saline waters in the lower Aromas Sand within the lower-aquifer system.
The precipitation data indicate that there were at least nine dry and nine wet periods that range from 2 to 19 years during the period of record, 1880–1997. The ground-water pumpage, runoff, streamflow and related water quality of streamflow also vary with seasonal and climatic periods.
Recharge occurs from deep percolation of precipitation and from infiltration of streamflow. Streamflow originates from local runoff and from outside the valley as inflow from the Pajaro River. Although partly regulated, streamflow in the Pajaro River at Chittenden is less than 200 cubic feet per second 88 percent of the time and is less than 12 cubic feet per second 50 percent of the time. Streamflow water-quality data suggest that there may be several sources of poor-quality water that contribute to elevated chloride, sulfate, and nitrate concentrations in streamflow. The poor water quality occurring during lower streamflows indicates that low flows may be an additional source of salinity for ground-water recharge as streamflow infiltration along the Pajaro River.
The geochemical data from this study indicate that the two main sources of recharge are deep percolation of local runoff and streamflow infiltration of Pajaro River water. The geophysical and geochemical data suggest that only the shallow alluvial aquifer and parts of the upper Aromas Sand that constitute the upper-aquifer system are being replenished by recent recharge in the coastal areas of Pajaro Valley and represent the renewable ground-water resources. These data also suggest that there is very little vertical flow through the layered aquifer systems in the coastal regions. The confining aquitards are laterally extensive but may be missing in places owing to fluvial erosion or offsetting by fault movement. Geochemical and geophysical data indicate that the ground water from some parts of the upper and lower Aromas Sand in the coastal regions was recharged thousands of years ago and may, in part, represent nonrenewable ground-water resources.
The analysis of major-ion chemistry, in combination with isotope and trace-element/chloride ratios, indicates that the coastal ground-water and surface-water samples can be described in seven groups. These groups represent recently recharged shallow aquifers (Group 1), shallower aquifers containing older water (Group 2), shallow aquifers containing recently intruded seawater (Group 3), deeper aquifers containing older seawater (Group 4), deeper aquifers containing older freshwater (Group 5), Pajaro River streamflow (Group 6), and local runoff from streams and sloughs (Group 7).
The ground-water levels have been near or below sea level at most coastal monitoring wells since their installation in the 1980s. Water-level changes include long-term decline, climate-period changes, and seasonal changes from agricultural pumping. The seasonal and long-term declines owing to dry years and increased pumpage from the lower Aromas Sand are similar to the water-level declines measured in other local deep wells since 1970. At the time of this study (1998–99), water levels in most coastal and some inland water-supply wells as far inland as Murphy Crossing in the Pajaro subarea and in the Springfield subarea were below the estimated water levels of 5 to 15 feet above sea level required to stop seawater intrusion.
The predominant source of the chloride in the coastal regions of Pajaro Valley may be recent seawater for aquifers in the older alluvium and the upper Aromas Sand and old seawater or connate water in the aquifers of the lower Aromas Sand. Geochemical and geophysical data indicate that saline water is vertically restricted for both the recent seawater intrusion in the older shallow alluvium and upper Aromas Sand and for the older seawater or connate water present in the lower Aromas Sand in Pajaro Valley.
Purpose and Scope
Approach to Investigation
Description of Study Area
Land and Water Use
Coastal Monitoring-Well Sites
Long-Term Water Levels and Seawater Intrusion
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry
Characteristics of Water from Monitoring and Supply Wells
Source, Age, and Movement of Ground Water
Seawater Intrusion and Saline Ground Water
Summary and Conclusions
Appendix 1. Summary Analysis of Geophysical, Lithologic, and Well-Construction Data Analysis for Selected Monitoring Wells in the Pajaro Valley
Appendix 2. Water-Chemistry Data for Selected Monitoring Wells and Surface Waters in the Pajaro Valley
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