ITEM:		INFORMATIONAL ITEM/STAFF REPORT
28.	SEMI-ANNUAL GROUNDWATER-QUALITY MONITORING REPORT
Meeting Date:			February 25, 2021	Budgeted:	Yes
From:			David Stoldt,	Program/	Hydrologic Monitoring 2.6
			General Manager	Line Item No.:	2-6-1 G, and 2-6-2 D
Prepared By:			Jonathan Lear/	Cost Estimate:	N/A
			Tom Lindberg
General Counsel Review: N/A
Committee Recommendation: N/A
CEQA Compliance: This action does not constitute a project as defined by the California Environmental Quality Act Guidelines Section 15378.

SUMMARY: Water-quality results from the Fall 2020 sampling of the Monterey Peninsula Water Management District’s (District’s) monitor well networks in the Carmel Valley aquifer and the coastal areas of the Seaside Groundwater Basin 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. Currently, collection of samples from the Carmel Valley monitor wells is conducted on an annual basis. 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. Beginning in 2007, the District was retained by the Seaside Basin Watermaster to collect water-quality samples from the District’s Seaside Basin coastal monitor wells on a quarterly basis. The results of that sampling are reported to the Seaside Basin Watermaster Board on an annual basis. Results of the Fall 2019 and Fall 2020 sampling of the Seaside Basin coastal monitor wells are included in this report.

DISCUSSION/ANALYSIS:

Carmel Valley Aquifer Monitor Wells - Results from the Fall 2020 sampling are provided in Exhibit 28-A. Six monitor wells in the lower Carmel Valley were sampled during Fall 2020, per the sampling schedule described above. Review of these water-quality results indicates that, in general, there are minor changes in overall water quality compared to samples collected in 2019 (provided here as a reference in Exhibit 28-B). A seventh well that was formally sampled in the Fall (16S/1E-13Md), was not sampled in Fall 2020 or Fall 2019 because it was submerged under high water in the Carmel River Lagoon wetlands during the sampling period. Another well that had been sampled during this period was destroyed by flooding in March of 2011 when the river scoured away the south end of the Carmel River State Beach parking lot. The locations of the sampling points are shown on the map in Exhibit 28-C. Changes in water quality for specific wells are discussed below.

Staff is particularly interested in tracking indicators of potential seawater intrusion in the coastal portion of Carmel Valley. Accordingly, three clustered 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 depth wells in the coastal area are subject to mixing of fresh water and saline water as high tides and surf overtop the sand berm between the lagoon and the ocean. This contributes to episodic mixing within the shallower and intermediate zones of the aquifer, but is not indicative of larger-scale seawater intrusion into the aquifer. All three wells in the cluster closest to the ocean were destroyed by river erosion in 2011, and all three of the wells in the next closest cluster to the ocean were inaccessible due to high water during the sampling period, so currently, only the deeper well at one of the three coastal locations is sampled.

Well 16S/1W-13Lc is the deepest in the array of three wells located on State Parks property near the Carmel Area Wastewater District treatment plant at River Mile (RM) 0.65, currently the most proximate well to the ocean in Carmel Valley that is available for sampling. There is an overall increasing trend in Specific Electrical Conductance (SEC) and Chloride from 1989 to 2020 (Exhibit 28-D) with some notable fluctuations. Both SEC and Chloride declined from 2006 to 2008. While there has been a generally upward trend since then, both constituents were lower in 2020 relative to 2019. Current Chloride concentrations remain below peak levels observed at this location in Water Year 2013, but SEC has also dropped below the 2013 value. Additional background on historical water-quality at the coastal monitor well sites can be found in District 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.

Well 16S/1E-23E4, located 6.53 miles upstream from the mouth of the Carmel River, has had fluctuating water quality in the past - primarily as variably elevated Iron and Manganese, likely attributable to flooding along the roadside where this well is located. Elevated Iron and Manganese concentrations are not unusual in Carmel Valley; four of the six wells sampled in Fall 2020 showed levels above the State Drinking Water Standards for these constituents, although the levels of Iron were lower in 2020 relative to 2019 for all four of the wells, and Manganese was lower in all but one of the four. Results indicate no significant changes to water quality here in 2020 relative to 2019. Staff will continue to monitor the site to ensure the wellhead is secure from surface-water sources.

Well 16S/1E-23La, located 6.72 miles upstream from the river mouth, does not show a significant change in 2020 relative to 2019, but a graph of SEC and Chloride is included to track long-term trends as was described in previous Board packet reports (Exhibit 28-E). This graph indicates a downward trend in both SEC and Chloride at this site; most other constituents were not significantly different in 2020 relative to 2019.

Seaside Groundwater Basin Coastal 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. The wells were completed in pairs to compare water from the two main aquifers in the basin, the deeper Santa Margarita formation, and the shallower Paso Robles. In 2009 District staff switched from air-lifting samples from wells in Seaside to “micro-purging”, which generally extends the well life. In Fall 2020, 10 dedicated monitor wells at six different sites were sampled. Results of water-quality sampling from 2020 and 2019 for the Seaside wells are provided in Exhibit 28-A and Exhibit 28-B, respectively. The locations of the Seaside monitor wells are shown on the map in Exhibit 28-F. Results for most constituents in most of the wells were not significantly different in 2020 relative to 2019, with few exceptions. SEC is higher in every well Seaside Basin well sampled in 2020 relative to 2019, and in a one (15S/1E-15N2 located about 400 feet from the ocean), the level is slightly over the lower drinking Water Standard of 900 ppm. In another (16S/1E-12Fa located about 6,000 feet from the coast) SEC and Chloride were both notably higher than in 2019, although both were below State Drinking Water Standards. Four of the wells in the Seaside Groundwater Basin showed levels of Iron above the Drinking Water Standard (0.3 milligrams per liter) in Fall 2020. 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

28-A Groundwater-quality Monitoring Results - Fall 2020

28-B Groundwater-quality Monitoring Results - Fall 2019

28-C Location of MPWMD Lower Carmel Valley Water-quality Monitoring Wells

28-D Water-quality Results in Well 16S/1W-13Lc in Carmel Valley

28-E Water-quality Results in Well 16S/1E-23La in Carmel Valley

28-F Location of MPWMD Seaside Basin Water-quality Monitoring Wells

28-G Water Quality Results for Wells 15S/1E-15N2 and 15S/1E-15N3 in Seaside

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