ITEM:

INFORMATIONAL ITEMS/STAFF REPORTS

 

29.

SEMI-ANNUAL GROUNDWATER QUALITY MONITORING REPORT

 

Meeting Date:

February 23, 2006

Budgeted:

Yes

 

From:

David A. Berger,

Program/

Hydrologic Monitoring 2.6

 

General Manager

Line Item No.:

2-6-1 G, and 2-6-2 D

 

Prepared By:

Joe Oliver/

Cost Estimate:

N/A

 

Tom Lindberg

 

General Counsel Approval:N/A

Committee Recommendation: N/A

CEQA Compliance:N/A

 

SUMMARY:Water quality results from the Fall 2005 sampling of the Monterey Peninsula Water Management Districtís (Districtís) monitor well networks in the Carmel Valley aquifer and Seaside Coastal Ground Water Subareas are presented and briefly summarized below.

 

BACKGROUND:The District has maintained a groundwater quality monitoring program in the Carmel Valley Aquifer since 1981, and in the Seaside Groundwater Basin since 1990. As part of the Districtís former Strategic Plan, the District conducted an assessment of the ground waterquality monitoring program in 1996.Based on review of long term data trends, it was determined that some modifications could be made to the sampling schedules in both the Carmel Valley and Seaside Basins without compromising the effectiveness of the program.Accordingly, collection of samples from the Carmel Valley monitor wells has been reduced from semi-annual to annual.The sampling schedule for Carmel Valley is staggered, with upper valley wells (i.e., upgradient of the Narrows), sampled in Spring and lower Carmel Valley wells in Fall, to coincide with the historically higher nitrate concentrations in these respective areas.Collection of samples from the Seaside Basin monitor wells was also reduced to once per year in Fall, coinciding with the historically low water levels in the basin at this time of the year.

 

DISCUSSION/ANALYSIS:Carmel Valley Aquifer Monitor Wells - Results from the Fall 2005 sampling are provided in Exhibit 29-A.Eight monitor wells in the lower Carmel Valley were sampled during Fall 2005, as per the sampling modification described above.The locations of these sampling points are shown on the map in Exhibit 29 -C.Review of these water quality results indicates that, in general, there are minor changes in overall water quality compared to samples collected in Fall 2004 (data reported in the February 24, 2005 Board packet and provided here in Exhibit 29 -B).Staff is particularly interested in tracking indicators of potential sea-water intrusion in the coastal portion of Carmel Valley.To that end, three sets of wells were established west of Highway 1, with each set being made up of three wells completed at different depths.Review of historical data indicated that the shallower and intermediate wells in the coastal area are subject to the mixing of fresh water and saline water as high tides and surf overtop the sand berm between the lagoon and the ocean, contributing to episodic mixing within the shallower and intermediate zones of the aquifer.Accordingly, only the deeper wells at each location are now being sampled.Well number 16S/1W-14Jg is the deepest of an array of three wells located at the Carmel River State Beach parking lot, 0.07 miles (approximately 375 feet) from the shoreline.Exhibit 29-E shows that specific conductance and chloride concentration increased in this well in Fall 2005 relative to Fall 2004, but these levels were not unprecedented.Note that the scales on the vertical axes of the graphs presented in this staff note are different to help discern trends in the data.Due to the proximity to the ocean and the permeability of the alluvial sediments, there has been mixing of fresh and seawater at this site, most notably during the end of the 1987 Ė 1991 drought period.Exhibit 29-F shows that specific conductance and chloride concentration also increased slightly in the deeper wells located 0.31 and 0.65 river miles from the shore (16S/1W-13Md and 16S/1E-13Lc, respectively) in Fall 2005 relative to Fall 2004.Again, these levels are not unprecedented, but there does appear to be a slight trend toward increased levels over time.The anomalously high specific conductance and chloride concentrations in well number 16S/1W-13Lc in Fall 2000 are suspicious.The higher values observed early in the sampling program are at least partially attributable to the fact that there was no fresh water inflow to the lagoon for approximately four years (April 1987 until March 1991).This situation is further discussed in Technical Memorandum 90-04, Summary of Carmel Valley Groundwater Quality from Coastal Monitor Wells, which is available at the District office.†† Staff will continue to track future results for trends that might indicate significant changes in concentrations of these or other constituents in the coastal area of the aquifer.

For the five wells located farther inland, changes in specific conductance ranged from a decrease of three percent at well number 16S/1E-23E4, located 6.5 river miles from the coast, to an increase of nearly 60 percent at 16S/1E-23La, located less than one quarter mile farther inland.Exhibit 29-G shows specific conductance and chloride concentration at well number 16S/1E-23La.The high chloride concentration in well number 16S/1E-23La in Spring 1993 is anomalous.Staff will continue to track future results for trends.

Seaside Coastal Subareas Monitor Wells - Since 1990, the District has been collecting water quality samples from coastal monitor wells in the Seaside Groundwater Basin, for the purposes of water quality characterization and sea-water intrusion monitoring.In Fall 2005, 12 monitor wells were sampled.Results of water quality sampling from Fall 2005 and Fall 2004 for the Seaside wells are provided in Exhibit 29-A and Exhibit 29-B, respectively.The locations of the Seaside wells are shown on the map in Exhibit 29-D.These results indicate little change from previous results over the period of record for the existing wells, and that there is no indication of sea-water intrusion in the two principal aquifer units -- the Paso Robles Formation (i.e., shallower unit) and Santa Margarita Sandstone (i.e., deeper unit) -- in this area of the Seaside Groundwater Basin at the present time.One well, well number 15S/1E-23Ca, that had shown an increase in specific conductance over the previous two years showed a decrease of 19 percent in Fall 2005 relative to Fall 2004.Results from this single well are not considered significant.This well is the shallower of a pair of monitor wells completed at Ord Terrace School.Results for specific conductance for the other wells in the basin were slightly lower in every well relative to Fall 2004, with one exception.Specific conductance for well number 15S/1E-15N3 was one percent higher in Fall 2005 relative to Fall 2004.No remarkable changes were detected in other constituent concentrations for any wells in the area.Staff will continue to track results for trends that might indicate significant changes in any wells in the basin.†† A more complete historical summary of the Seaside Basin coastal groundwater quality data is contained in District Technical Memorandum 97-02 Seaside Basin Coastal Monitor Wells: Ground Water Quality Monitoring Results, 1990-1996, which is available at the District office.

 

EXHIBITS

29-A††† Ground Water Quality Monitoring Results - Fall 2005

29-B††† Ground Water Quality Monitoring Results - Fall 2004

29-C††† Location of MPWMD Carmel Valley Water Quality Monitoring Wells

29-D††† Location of MPWMD Seaside Basin Water Quality Monitoring Wells

29-E††† Specific Conductance in a Coastal Well in Carmel Valley

29-F†††† Specific Conductance in Two Wells in Carmel Valley

29-G††† Specific Conductance in Well 16S/1E-23La in Carmel Valley

 

 

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