Parent Layer: Stormwater and Sanitation

Name: EH Septic Shallow Groundwater

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Description: Presence of shallow groundwater is one of the most significant potential constraints to proper performance of onsite wastewater treatment systems (OWTS, aka septic systems) in Santa Cruz County. Effective treatment of wastewater is promoted by unsaturated soil conditions and presence of shallow groundwater can lead to transport of untreated effluent with potential contamination of surface water and groundwater. Proper design of an OWTS requires a determination of the expected depth of groundwater and design of the dispersal to maintain an adequate separation of unsaturated soil between the bottom of the system and groundwater. Where adequate separation cannot be provided, enhanced treatment components may be required to reduce contaminants prior to discharge of effluent to the soil. Requirements for groundwater separation are specified in SCC 7.38.150(B)(9).Depth to shallow groundwater fluctuates significantly in Santa Cruz County based on rainfall amounts and topography. The depth may fluctuate as much as 20 feet in a given location from summer to winter. Because of this wide fluctuation, observation of shallow groundwater depth for the purposes of designing an OWTS can only be done during wet winter conditions, when at least 60% of the mean annual rainfall has occurred and at least 6 inches of rain has fallen in the previous 30 days (SCC 7.38.120(B).Given the limited amount of time in any given year that observations of shallow groundwater can be made, it is important to have good mapped information to indicate where shallow groundwater may occur and at what depth that be expected to occur. This is particularly important when designing repairs of failing OWTS during the summer months and other situations where groundwater depth needs to be estimated. Environmental Health currently has some record of observed or estimated shallow groundwater depth for almost 8000 parcels in the County. In addition, in 1986-88 EH installed some 50 boreholes and 20 shallow monitoring wells to monitor shallow groundwater levels in communities of the San Lorenzo Valley. Monitoring of some shallow wells continues to determine current conditions relative to historical levels.

Service Item Id: e60a3acd43094f56a5be4bd5a47e2e8c

Copyright Text: Potential Shallow Groundwater: Countywide locations and soils likely to experience shallow groundwater were mapped on USDA soil maps in 1977 by County Water Quality Chemist Robert Aston as part of the Growth Management Plan Environmental Report (1977), with input from Environmental Health District Specialists. Shallow Groundwater Likely: Depth of likely shallow groundwater in San Lorenzo Valley communities was mapped based on data from 50 boreholes drilled in 1986, winter observations of groundwater in boreholes and monitoring wells, and Environmental Health file records of winter groundwater observations related to applications for onsite wastewater treatment systems. Maps were drawn by Environmental Health staff, John Ricker and Tom Graves, based on recorded groundwater observations and local topography. The indicated range of likely groundwater depth is the depth that is expected to occur approximately 10% of the time. Ninety percent of the time during an average year groundwater depth would be expected to be deeper than indicated. For more information on OWTS performance and shallow groundwater see the Santa Cruz County Local Agency Management Program for Onsite Wastewater Treatment Systems (LAMP: https://www.scceh.org/NewHome/Programs/LandUse/LocalAgencyManagementProgram.aspx) Data on shallow groundwater depth from the Environmental Health files is available in the layer EH Septic Site Extract, and can also be accessed from individual parcel files: https://sccdocs.santacruzcounty.us/ENV/CustomSearch.aspx?SearchName=SepticandWaterWells&repo=EnvironmentalHealth&cr=1

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