County releases Phase III of Midcoast Groundwater Study

By on Fri, June 11, 2010

The Midcoast is divided into 23 groundwater sub-areas, each with its own characteristics. Click for larger version.

San Mateo County has released the report on the third phase of its Midcoast groundwater study.

We will release an analysis of the report in the near future. You can read the conclusions and recommendations after the jump. Or you can download the report from Coastsider and draw your own conclusions.

The Phase II report released last year raised the alarm of risk of saltwater intrusion due to overpumping in some areas of the Midcoast.


Through a variety of methods to contact well owners on Midcoast San Mateo County in a ‘call for wells’ campaign, we identified 20 wells that were not in use.  Fourteen wells were suitable for monitoring with a continuous-recording datalogger, and ‘spot’ manual depth-to-water measurements were conducted in the other six wells.  Monitoring well data were also gathered from three California Department of Water Resources (DWR) monitoring wells, from shallow monitoring wells at Leaky Underground Storage Tank (LUST) remediation sites, and from Montrara Water and Sanitary District and Coastside County Water District wells records.  We also gaged flow at 11 stations on Midcoast streams – Martini Creek, Daffodil Canyon, Montara Creek, San Vicente Creek, Denniston Creek, Deer Creek, and Arroyo de en Medio.  Three of the streams had paired gaging stations to assess baseflow recharge to the underlying terrace. Findings of our surface water and groundwater monitoring efforts are reported for the monitoring period September 2009 through February 2010, principally to assess baseflow during the third year of drier-than-normal rainfall and early winter recharge.

Rainfall and runoff records indicate that the drought that began in water year 2007 is currently less severe the previous multi-drought from water years 1987 to 1992, or any consecutive three years of that drought.  Sparse but valuable long-term groundwater monitoring data from DWR monitoring wells show water level decline during the previous drought was equal to (in the Airport Subarea) and exceeding (in the Frechmans Terrace Subarea) the decline during the extreme 2-year drought of 1976 to 1977.  Groundwater level decline during 2007 was equally low but had since recharged moderately.  Baseflow gaging confirmed flows during 2009 are similar to drier-than-normal water year 2004 but not as low as during the previous drought or during the 1976 to 1977 drought.

Groundwater recharge from streams is significant and generally provides a hydraulic floor to water-level decline during the dry season, moderating seasonal fluctuation.  Streams on the Midcoast generally have a considerable depth of alluvium that allows storage of groundwater recharge.  We recorded significant recharge through the terrace reaches of Denniston Creek and Deer Creek.  Arroyo de en Medio, San Vicente Creek, and Dean Creek remained dry during the dry season while having a shallow water table present within bed sediments.  The gaging stations on these streams showed flow following the first major winter storm, which occurred on October 13, 2009.

Wells near creeks showed water levels related to stream recharge and monitoring wells at distance from stream courses and at higher elevations such as in Montara and Upper Moss Beach had deeper water levels.  These areas largely rely on recharge from direct rainfall. Pumping from wells at distance for creeks generally show greater drawdown effects, and in these local areas drought constraints should be first identified.  Limited data show marginally lower groundwater levels than pre-drought levels in upland areas.

Groundwater storage seems adequate but not without local variability. Groundwater data indicate that wetland areas and the Pillar Point Marsh appear to have been unaffected by the drought. This is evidenced by high groundwater in wetland areas and artesian conditions at the marsh.  Groundwater elevation data indicate that conditions for sea- water intrusion have not developed.  The most convincing data are from the piezometers located in the Pillar Point Marsh and wells in the Granada Terrace near the coast.


A groundwater monitoring program should be part of protecting and managing any aquifer system which is a primary source of water supply.  San Mateo County has tried on more than one occasion to implement a monitoring program for the Midcoast. [FOOTNOTE: A specific monitoring program was adopted as a mitigation measure during approval of the 1989 Montara-Moss Beach water well EIR.  County staff made two initial but unsuccessful attempts to implement the program during the subsequent years.  Similar recommendations were developed as part of the 1992 Airport Aquifer and 1988 El Granda investigations.]  The set of wells and stream gages that were emplaced for this study can—with some refinement—serve as the core of such a monitoring program.  If so, the present study has shown that additional wells are needed in areas of poor coverage, in areas of limited recharge, and in areas of variable groundwater condition, particularly in upper terrace areas, such as Montara Subarea, Upper Moss Beach, El Granada, Miramar, and Seal Cove.  Developing storm recharge programs for these areas would benefit local groundwater supplies – such as recharge from rooftop runoff and from storm-water ponds, particularly from late-season storms, as well as rainwater harvesting.

A sub-basin water balance model and multi-year drought analysis was developed in the Phase II study for El Granada, Moss Beach and Miramar Sub-basins.  The Phase II study appropriately recommended additional monitoring and model calibration.  A water balance model and drought analysis should also be developed for the other sub-basins.  The water balance models would be a better predictive tool if further calibrated with a focused stream gaging program and long-term monitoring of key wells.  These data could also support an adaptive management program.  A subset of rain and stream gages and wells can be readily equipped to provide web-based real-time data, providing an effective tool to better understand, track, and respond to changing groundwater conditions, making the Midcoast’s water supply more reliable, resilient, and also better known to the community that it sustains..

One of the main constraints to an effective water-balance model is lack of local evapotranspiration data.  At present, evapotranspiration is estimated from state publications based on data collected at sites in different settings.  The Airport Terrace is an ideal location for regional reference evapotranspiration (ETo) monitoring.  The California Irrigation Management Information System (CIMIS) should operate a station here to measure ETo for the Midcoast, which would assist with calibration of all water balance calculations as well as guide stream, wetland, and lagoonal habitat management on the Midcoast and South Coast.

A conceptual model for each sub-basin and a groundwater flow model would assist groundwater management.  We have provided the beginnings of this process in this report, helping to identify and simulate drought-year conditions as essential basin objectives, and a reasonable common first step towards developing a groundwater management plan.  This would be an effective next step in the Midcoast ground planning.

5.1 Specific Recommendations by Subarea (discussed in Section 3.2)

  • Additional monitoring wells at distance from stream courses are needed in El Granada. Gaging Deer Creek successfully quantified baseflow during 2009 and should continue during baseflow 2010.  Results could be used to calibrate the water balance model performed during the Phase II study.  A sub-basin groundwater flow model would assist groundwater management.  A local recharge program would benefit local groundwater supplies.
  • Additional groundwater monitoring is needed in the Arroyo de en Medio Terrace and Frenchmans Terrace, especially at distance from the creeks.  Additional gaging of Arroyo de en Medio is needed to confirm groundwater recharge trends and to quantify persistence of baseflows, with results used to calibrate a water balance model.
  • A water balance model, drought analysis, and a groundwater flow model would assist groundwater management of the Airport Terrace.  Additional analysis should include developing dry-season groundwater contour maps to compare with those reported during the 1987 to 1992 drought (LSCE & ESA, 1992, 1991, 1987).  Gaging Denniston Creek would greatly assist calibration of the models.  In addition, the Airport Terrace is an ideal location for regional reference evapotranspiration (ETo) monitoring, which would assist with calibration of all water balance models on the Midcoast.
  • Additional groundwater monitoring is needed in Moss Beach, particularly at distance from the creeks and in Upper Moss Beach.  Additional gaging of San Vicente Creek is needed to confirm groundwater recharge trends and quantify baseflow persistence; results could be used to calibrate the Moss Beach Sub-basin water balance model performed during the Phase II study.  A closer look at gaging Dean Creek is justified.  A calibrated groundwater flow model would assist groundwater management, and developing a local recharge program, particularly for Upper Moss Beach, would benefit local groundwater-storage evaluation and drought readiness.
  • Developing storm recharge programs would benefit local groundwater supplies in the Montara and Ocean View Farms terraces.  Additional groundwater monitoring wells are needed, especially in the upper portion of Montara Terrace.  A sub-basin groundwater flow model would also assist groundwater management.
  • Additional groundwater monitoring and a local recharge program are justified in Portola and Wagner Valley.
  • Installation of a dedicated monitoring well in the Seal Cove Subarea would provide local groundwater data and assist groundwater management of this subarea.