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ITEM:
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INFORMATIONAL
ITEMS/STAFF REPORTS
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29.
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SEMI-ANNUAL GROUNDWATER
QUALITY MONITORING REPORT
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Meeting
Date:
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February 23, 2006
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Budgeted:
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Yes
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From:
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David A.
Berger,
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Program/
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Hydrologic Monitoring 2.6
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General
Manager
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Line Item No.:
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2-6-1 G, and 2-6-2 D
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Prepared
By:
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Joe
Oliver/
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Cost Estimate:
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N/A
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Tom Lindberg
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General Counsel Approval: N/A
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Committee Recommendation: N/A
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CEQA Compliance: N/A
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SUMMARY: Water quality results from the Fall 2005
sampling of the Monterey Peninsula Water Management District’s (District’s)
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 District’s 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 – 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