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2001 Hydrologic Monitoring Report
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2 Findings and Recommendations
- Three major aquifers are identified in the vicinity of Sequim and its water supply sources. They are commonly referred to as the Shallow, Middle, and Lower aquifers. The Dungeness River is the largest stream in the area. In the vicinity of Sequim (east of the river), smaller streams are typically fed by groundwater discharge and irrigation tailwater.
- The City's three groundwater supply sources include: the Dungeness River Infiltration Gallery (Shallow Aquifer), the Silberhorn Wellfield (Shallow Aquifer), and the Port Williams Wellfield (Lower Aquifer). The City is actively shifting the bulk of its pumping withdrawals from the Infiltration Gallery and Silberhorn Wellfield to the Port Williams Wellfield.
- Over the past 24 years (1978 to 2001), pumpage from the City's water supply sources has increased from about 0.4 million gallons per day (mgd) to about 0.8 mgd. Most of this increase occurred between 1982 and 1989, followed by a gentle increasing trend through present. Pumping from the City's two wellfields is estimated to have increased by about 0.38 mgd between 1990 and 2001 due to reduced withdrawals from the Infiltration Gallery. Because the City's service area is predominantly sewered, this pumping increase is predominantly associated with consumptive water use. However, a portion of the increased pumpage recharges the Shallow Aquifer as return flow from residential irrigation. The City estimates that it supplies about 30 percent of the water users on the Sequim-Dungeness Peninsula east of the Dungeness River.
- Additional pumping from newly constructed domestic wells and other water supply systems contributes to the total change in groundwater withdrawals over time. Pumpage from domestic wells in the Sequim vicinity is estimated to have increased by at least 0.58 mgd between 1990 and 2001, of which about 0.32 mgd is estimated to be used consumptively. The remaining portion of the increased pumpage recharges the Shallow Aquifer via septic effluent and irrigation return flow. Based on population growth, significant pumping increases are similarly expected for public and private water systems; however, data were not readily available to support a quantitative estimate of this increase. The City currently reuses a portion of its reclaimed municipal wastewater for irrigation and flow augmentation in Bell Creek. Bell Creek flow augmentation is permitted by Washington State Departments of Ecology and Fish & Wildlife.
- The City's hydrologic monitoring program includes: groundwater levels in monitoring wells located on the City's wellfields and in private wells surrounding the wellfields; climatic monitoring, water quality analyses from drinking water sources; water quality from reclaimed wastewater; water quality from Bell Creek. Monitoring data collected by Clallam County, Washington State Department of Ecology (Ecology); the U.S. Geological Survey (USGS), and the Jamestown S'Klallam Tribe have also been incorporated into this report.
- Review of precipitation data reveals no long-term trends, however series of wet years and dry years do occur. Most recently, low rainfall in the Sequim vicinity is noted in the early 1990's, 2000 and 2001. Below average snowpack in the Olympic Mountains (which supply the Dungeness River) is noted between 1998 and 2001.
- Annual agricultural diversions from the Dungeness River have reduced over the last two decades from about 115 cfs in 1979 to about 55 cfs in 2000. This reduction is largely due to: lining of ditches and laterals; improvement of on-farm irrigation efficiencies; replacement of pasture crops to crops with lower water demands; and reduced diversions for stock watering. Diversions were substantially reduced in 2001 due to insufficient flow in the Dungeness River caused by below average snowpack.
- Groundwater levels in the vicinity of the City's Port Williams Wellfield have shown notable declining trends over the period of record (1996 thru present). Rates of decline are similar in all three aquifers (approximately 1-2 ft/year); however, seasonal variations are greatest in the Lower Aquifer where the City's production wells are completed. The data indicate that pumping from the Lower Aquifer affects groundwater levels in the Middle Aquifer, but that the Shallow Aquifer is somewhat insulated from seasonal water-level variations associated with Port Williams pumping. Water levels in all three aquifers are affected by a variety of factors, including: pumping by the City, smaller water systems and private wells; long-term changes in recharge from irrigation; and short-term changes in recharge from precipitation. In addition, trends observed in one aquifer may cause similar trends in an underlying or overlying aquifer due to hydraulic connection through intervening aquitards. Pumping from the Port Williams Wellfield likely contributes to the observed groundwater level declines; however, distinction between the City's pumping, other groundwater withdrawals, and changes in irrigation recharge is a complex task that requires analysis beyond the scope of this report.
- Groundwater levels in the vicinity of the City's Silberhorn Wellfield have similarly shown declining trends over the monitoring period (1993 thru present) as well as since the wellfield was constructed in 1975. The most significant declines are reported to have occurred in the early 1990's. Since construction of the Port Williams Wellfield in 1996, the City has reduced pumping withdrawals from the Silberhorn Wellfield. However, groundwater levels have continued to decline. Local groundwater levels appear to be particularly responsive to irrigation recharge during the growing season. Irrigation managers report little change in summer diversions or ditch conveyance losses (e.g. tightlining) over the past several decades; however, winter stockwater diversions have declined. In addition, the number of wells in the Silberhorn vicinity increased by 140 percent over the last 20 years. Further evaluation of changes in irrigation recharge and increased pumping by other groundwater users would be required to better identify the cause of water-level declines observed near the Silberhorn Wellfield.
- Where present, the observed groundwater level declines will have only minor effects on pumping yield for wells completed in the Lower Aquifer for most water users due to the relatively large pumping drawdowns available in Lower Aquifer wells. However, the observed rates of decline could have significant effects on pumping yield for some wells completed in the Shallow Aquifer, particularly shallow wells with little penetration below the water table. In the Middle Aquifer, the degree to which observed water-level declines affect well yield will depend on the degree to which the available water column is used for drawdown, the efficiency of the well, and the permeability of the aquifer. If groundwater level declines continue at recently observed rates, pumping yield reductions will be exacerbated in selected wells.
- Water quality from the City's sources of supply is excellent, with concentrations of drinking water constituents typically well below the State's drinking water standards.
- Water quality in Bell Creek, both upstream and downstream of Carrie Blake Park, shows seasonal exceedence of the Class A standards for temperature and dissolved oxygen in the summer and early fall. Fewer exceedences are noted prior to the summer of 2001; however data are relatively sparse over the earlier period of record (1997 thru 2000). Phosphorus concentrations are elevated both upstream and downstream of the park, and may reflect contamination by livestock, other mammals, birds or failing septic systems. Class A standards for fecal coliform are regularly exceeded. Ammonia values show an increasing trend over time, but the laboratory results are inaccurate due to chemical interference with aqueous iron naturally occurring in the creek. The City will be revising its laboratory method to eliminate this source of analytical error.
- The system for streamflow augmentation for Bell Creek was first tested in August 2001, and put into long-term operation in December 2001. The reclaimed water is introduced to the stream in Carrie Blake Park. The majority of sampling events reflect background water-quality conditions, with only two sampling rounds collected after augmentation began. In general, too few data are available to indicate water quality trends associated with the streamflow augmentation. Additional data collection from monitoring points upstream and downstream of the park will be required to evaluate possible effects. While ammonia data suggest an increase from upstream to downstream, but inaccuracies in the analytical method require improved sample analysis to establish whether such a difference actually occurs.
- Recommendations for modifications to the City's monitoring program and further data analysis are presented in Section 11 of this report. The following items summarize PGG's recommendations:
- The frequency of water-level measurements in the City's wells should be modified to improve data resolution and make the best use of data automatically recorded by computer datalogger. Overall, these modifications will result in a slight reduction in effort required of the City's staff but will maintain the quality of the data. Operation of the existing datalogger should be maintained to continue collection of high quality data.
- Groundwater level monitoring in private wells, conducted by the City and other agencies, should be expanded to include more wells in the Middle and Lower Aquifers. Data sharing between agencies should continue, and coordination of monitoring activities should occur whenever possible. Private residents and water systems with relatively deep wells should be encouraged to provide access for water-level measurement or share data they collect themselves. Monitoring of electrical conductance or chloride is recommend for wells completed in the Middle or Lower Aquifers along the coast.
- The City should continue to enter digital data (currently stored on hard-copy) into spreadsheets provided by PGG on a regular basis.
- The City's system of site identification for monitoring stations along Bell Creek should be integrated to avoid differing site ID's for the same stations. If ongoing Bell Creek sampling is expected to generate large quantities of data, the City should consider storing the data in a centralized database. Minor modifications to the parameters and procedures associated with the City's Bell Creek monitoring program are further suggested in Section 11.
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