County releases Phase III of Midcoast Groundwater Study

Posted 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.


We had to wait years for a report with vague non-committal conclusions and a recommendation of “more monitoring is needed”?

I guess the County had a hard time locating a consultant who wouldn’t write a report that says “Immediately STOP allowing all these private wells.”

The power of accurate observation is commonly called cynicism by those who have not got it. - George Bernard Shaw (1856-1950)

Leonard we’ve had lots of rain this year. Are you really surprised with this report…or just being cynical?

Is the aquifer half empty ..or half full?

Cid, one wet year doesn’t matter. The question is whether the system can survive a sustained drought. 

We don’t know when that will happen, but we know that it will happen.

We don’t know if what will happen to the groundwater supply when it does happen. But the Phase II report said there was a risk of saltwater intrusion in some sub-areas of the Midcoast during a sustained drought.

Who, me, a cynic?  I trust you have proof.

Seriously though, the fundamental issue in the groundwater evaluation is the one that was not addressed and that the County doesn’t want addressed.  Who cares if there’s enough water now?  So what if water levels in the aquifer are basically stable now?  What the community needs to know and deserves to know is what the water levels are likely to be in a future with double or triple the number of wells as currently exist.

Here’s an easy to understand analogy.  Picture a jug of water.  Water is being poured in regularly, but not at a continuous rate.  Ten people have straws and are sucking water out at an essentially continuous rate.  So the water level goes up and down depending on whether the water input rate is ahead or behind the rate it’s being withdrawn.  If the input rate goes down for too long, the jug could run dry and then everyone loses.  That hasn’t happened.  Yet.

But now suppose that you allow ten or twenty more people to put straws in and start sucking water out.  The system was in balance with ten straws.  But if you’re now withdrawing water at double or triple the rate that you were when it was in balance, anyone who passed 1st grade math should be able to see that the level will drop over the long term since the long-term average input rate is stable.

Before anyone argues that my assumption of double or triple the number of straws is invalid, here are the dots to connect:
- Unincorporated Midcoast currently has 4000 or so houses.
- 1100 or so of those houses are currently on individual wells.
- County says we’re at 50% buildout.  That means that another 4,000 houses could be built under the existing zoning / land use plan.
- The two water districts have no water available for new connections, and may or may not ever have any.  A good bet is “never”.  (If there was more water to be had, surely the pro-growth CCWD Board would have found it by now.)
- Therefore, one should assume that a minimum of “most” of those additional 4,000 houses will be built on wells.
- That means that my estimate of double or triple looks low, it could be 5 times as many straws.
- How can anyone claim with a straight face that this isn’t a recipe for disaster?

Oh, and by the way, every new house that’s built covers up ground that currently allows groundwater recharge, so at the same time that new house on a well is taking more water out of the ground, it’s causing less water to into the ground.  And then more people complain about flooding and demand that more water be quickly transported away, leaving even less to soak into the ground.

I see your point. Too bad the County is hell-bent on building on all the empty lots on the midcoast so they can raise their tax revenues to pay for all the pensions at the Couunty Government. There are very few in-fill lots left vacant over the “hill” so we are their natural target.

I did read this in the report.
“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.”

    I purchased a rain-water harvesting system for my home vegetable garden this year. I am trying to be more sustainable, and not use the purified (treated) water that I am paying dearly for from MWSD to simply water my garden. That water is stored after it rains and eventually recharges the aquifer as if passes through my vegetable beds first.
  When larger homes are built on vacant lots there seems to be more run-off from the impervious surfaces of the structures. So I’m not too certain that your lot coverage idea is valid. (You probably have a great argument of course and I would love to hear it.)
    For one new home built near me on a swampy portion of land the owners even felt it neccessary to install a “dissapator system” as well as pervious pavers etc., but the run-off was still considerable creating quite the muddy situation on a nearby paper street. That street was always muddy in winter but with the construction excavation and the moving around of large berms of excavated dirt piles, I feel it became even muddier this season…in part because of all the extra rain and the earth-moving & bulldozing that preceeded it.
  I hope the drought is over for awhile!