The City's program of groundwater level monitoring is designed to observe water-level trends in the vicinity of their wellfields and in surrounding outlying areas. Groundwater levels are monitored in the City's own monitoring and production wells, as well as private wells in the wellfield vicinities. Monitored wells are completed in the Shallow, Middle and Lower aquifers.

Water-level trends and variations are observed in all three aquifers. Groundwater levels are affected both seasonally and from year to year by changes in both aquifer recharge and discharge. Aquifer recharge originates from infiltration of incident precipitation and by seepage losses from irrigation ditches and field applications. While recharge from rainfall occurs largely during the fall and winter months, recharge from irrigation is more substantial during the summer months. However, in the Middle and Lower aquifers seasonal patterns in groundwater recharge are typically more muted. Short-term variations in recharge may occur from several contiguous years of high or low rainfall; however, a longer-term trend exists in the form of agricultural water use conservation. The lining of ditches, increased on-farm irrigation efficiencies, use of lower water-demand crops, and reduction of winter stockwater diversions occurring over the last 15-20 years have resulted in reduced agricultural recharge to the Shallow Aquifer. Consequently, some degree of recharge reduction is expected for the Middle and Lower aquifers.

Changes in aquifer discharge originate predominantly from changes in pumping withdrawals. In the area of interest, population growth has resulted in increased water use by the City of Sequim, domestic wells, and is similarly inferred for other public and private water systems. Since construction of the Port Williams Wellfield in 1996, the City has actively shifted its pumpage away from the Infiltration Gallery and Silberhorn Wellfield towards production wells in the Lower Aquifer at Port Williams. Reduced Infiltration Gallery withdrawals leave more flow in the stream to benefit fish habitat. Reduced pumping from the Silberhorn Wellfield is beneficial for two reasons: 1) the production wells are completed in the Shallow Aquifer and therefore may influence baseflows in the nearby Dungeness River; and 2) the Shallow Aquifer was showing declining water-level trends in the early 1990's and corresponding yields in both the Silberhorn production wells and surrounding domestic wells were also declining. As discussed below, groundwater levels in the wellfield vicinities have responded to changes in pumping withdrawals as well as other factors.

8.1 Groundwater Level Trends in the Port Williams Wellfield Vicinity
The City's monitoring program in the Port Williams Wellfield vicinity includes water-level measurements in both Port Williams production wells, two City monitoring wells at the wellfield, and (originally) 6 private wells with distances from the wellfield ranging from 600 feet to 1.5 miles. The private wells (PD-7 through PD-12) are listed on Table 1 and shown on Figure 10. They are completed in all three major aquifers as well as the underlying undifferentiated deposits. In the past four years, access to wells PD-9, PD-11 and PD-12 has been denied by the well owners. In the Shallow Aquifer, wells monitored in the Port Williams Vicinity include the City's MW-1 and private wells PD-7 and PD-8. In the Middle Aquifer, wells monitored in the Port Williams vicinity include the City's MW-3 and private wells PD-9 and PD-10. In the Lower Aquifer, monitored wells include the City's production wells (PW-1 and PW-2) as well as private well PD-11. Well PD-12, completed in the undifferentiated deposits, is now monitored by the well owner; however, data were not provided for analysis at the time of writing this report.

For the purpose of analyzing water-level trends in the three major aquifers, charts were constructed to allow comparison of groundwater levels to annual pumping, monthly pumping, and annual precipitation. Figure 13 presents groundwater level trends in the Lower Aquifer, from which the City withdraws groundwater at the Port Williams Wellfield. The water levels presented from PW-1 represent "daily high" values collected by the datalogger during non-pumping periods, and are likely influenced by recovery from daytime pumping withdrawals. The groundwater levels presented from PD-11 are hand measured during daytime hours, and may reflect interference drawdowns when the wellfield is in operation (PD-11 is located 600 feet from PW-1). Despite possible pumping influences, trends in measured high groundwater levels are indicative of aquifer conditions⁶ . The groundwater levels shown on Figure 13 clearly respond to periods of heavy pumping, with seasonal declines noted in the summers of 1996, 1998, 1999, 2000, and 2001. The magnitude of seasonal decline generally corresponds to the intensity of summer pumping. On an annual basis, greater depths to groundwater correspond to higher average annual pumping rates. An overall increasing trend in Port Williams pumping between 1996 and 2001 corresponds to an overall decreasing trend in seasonal groundwater level highs.

The response of the Lower Aquifer to Port Williams pumping is consistent with basic hydrogeologic principals. In an undeveloped state, an aquifer will be in hydraulic equilibrium with the rest of the hydrologic system. When new pumping or changes in recharge affect the aquifer, water levels will adjust until a new equilibrium is reached. Pumping will cause groundwater levels to decline, whereas additional recharge will cause them to rise. Water-level declines associated with pumping will cause increased hydraulic gradients toward the pumped aquifer and increased inflow to the pumped aquifer from adjacent aquifers. When the increased inflow equals the rate of pumping withdrawal, water levels in the pumped aquifer will stabilize. Annual pumping in the Port Williams Wellfield has varied from year-to-year and shows an overall increasing trend (Figure 13). Groundwater level changes in the Lower Aquifer associated with Port Williams pumping will stabilize once the City's annual withdrawals become relative constant from year to year.

Observed groundwater level trends in the Lower Aquifer show about 10 feet of decline over the past six years, with smaller values expected with increasing distance from the wellfield. To put this magnitude of decline into perspective, it should be noted that Lower Aquifer wells in the Port Williams vicinity generally have water columns of over 200 feet. Reductions in available drawdown on the order of 10 or 20 feet are unlikely to have significant impacts on well yields at the pumping rates common for private wells and smaller water systems.

While local water-level trends in the Lower Aquifer are clearly affected by the City's withdrawals, other factors also affect observed aquifer responses. For example, in 1997 the wellfield was not pumped for 7 months between March and September, during which time groundwater levels exhibited a gradual decline. If Port Williams pumping were the only factor affecting groundwater levels, a rising trend would be expected due to aquifer recovery. The minimum 1997 water level occurred in August during the height of seasonal water demand, and therefore appears to be caused by the pumping of other water users. In addition, groundwater levels in the Middle Aquifer may affect those in the Lower Aquifer, and those in the Shallow Aquifer may affect the Middle Aquifer. Similarities and differences in trends observed between adjacent aquifers, as well as the multiple causes potentially affecting groundwater levels, are further discussed in the following paragraphs.

Groundwater trends in the Middle Aquifer near Port Williams Wellfield are shown on Figure 14. Wells monitored by the City include MW-3 (located on the wellfield), PD-9 (located about 0.8 miles east), and PD-10 (located about 0.9 miles west). In addition, Figure 14 presents hydrographs from two wells monitored by Ecology. The wells are included to allow comparison with trends on other parts of the Peninsula. Well EW-1 is located 2 miles southeast of the Port Williams Wellfield, and Well EW-8 is located 5 miles from the wellfield on the west side of the Peninsula (Figure 10). Similar water-level trends are observed in wells MW-3, PD-9 and PD-10. These Middle Aquifer trends are also comparable to those observed in the Lower Aquifer with similar timing of seasonal water-level variations, similar water-level declines over the 6-year record, but smaller ranges of seasonal water-level variation. Figure 15 presents a direct comparison of average rates of water-level decline in wells from all three aquifers located in the vicinity of the Port Williams Wellfield. In onsite wells, PW-1 (Lower Aquifer) shows a period-of-record decline of 1.7 ft/yr compared to 2.2 ft/yr in MW-3 (Middle Aquifer). Farther from the pumping center, Middle Aquifer Well PD-9 shows an average decline rate of 1.1 ft/yr. It should be noted that the decline rate estimated for Production Well PW-1 may be affected by "noise" in the water-level data due to recovery from periods of pumping.

Farther from the wellfield, Ecology Well EW-1 also shows a similar trend to MW-3, PD-9 and PD-10, although its recent dataset is relatively sparse. However, more distant Well EW-8 shows little similarity, with relatively modest water-level decline and data too sparse to compare seasonal variations. Given the well's distance from the Port Williams area, differing trends are not surprising.

The similar trends observed in the Port Williams vicinity for the Middle and Lower aquifers demonstrate a hydraulic connection between the two units. If Port Williams pumping is the dominant cause of water-level decline, both seasonal variations and long-term declines would be greatest in the wellfield vicinity and in the Lower Aquifer. The observed smaller range of seasonal variations in the Middle Aquifer is consistent with this understanding; however, estimated average rates of decline are steeper in the Middle Aquifer than the Lower Aquifer (possibly a result of noise in the data from Production Well PW-1). The rate of decline within the Middle Aquifer decreases by 50 percent from trends observed at the wellfield to those observed 0.8 miles to the east. Overall, the data suggest that Port Williams pumping plays a significant role in causing local drawdowns in both the Lower and Middle aquifers. However, the overall trend of declining water levels may well be affected by pumping from both aquifers, as well as pumping and changes in recharge to the Shallow Aquifer. As noted in Section 4.2, significant increases have occurred in groundwater withdrawals by domestic wells and are similarly expected for other (e.g. smaller) water systems.

Groundwater trends in the Shallow Aquifer near the Port Williams Wellfield are shown on Figure 16. Wells monitored by the City include MW-1 (located on the wellfield), PD-7 and PD-8 (both located about 0.4 miles west). In addition, Figure 16 presents hydrographs from 6 wells monitored by Ecology in widely varying locations on the Peninsula (see Figure 10). Similar water-level trends are observed in wells MW-1, PD-7 and PD-8⁷ . The three wells show very little seasonal water-level variation and show estimated declines ranging from 1.3 ft/yr in MW-1 to 1.7 ft/yr in PD-7 (see Figure 15). Most of the Ecology monitoring wells show much gentler rates of decline (e.g. 0.2 ft/yr), with wells located along the Dungeness River (EW-5 and EW-7) showing negligible declines. Water levels in these wells are likely supported by the stage of the river, whereas other wells are more influenced by changes in pumping and recharge. None of the wells show exacerbated declines associated with low rainfall in years 2000 and 2001.

Although water-level trends in the Shallow Aquifer show similar declines over the 6-year period of record to those observed in the Middle and Lower aquifers, they do not indicate similar influence from Port Williams pumping. Aquitards between the aquifers tend to dampen the influence of one aquifer on an adjacent aquifer, especially on the timescale of seasonal variations. However, steady declines in a single aquifer can propagate to an adjacent aquifer because the longer timescale allows gradual adjustment of the neighboring aquifer. Available data from the Shallow Aquifer show varying rates of water-level decline at different locations on the Peninsula. The highest recorded rate occurs 0.4 miles west of Port Williams Wellfield, with a slightly gentler rate observed at the wellfield. While declines may be affected by Port Williams pumping, the varying geographic distribution suggests that other factors are also influencing declines in the Shallow Aquifer.

The USGS analyzed water-level trends in the Shallow and Middle aquifers between the late 1970's and the mid 1990's (prior to Port Williams pumping), and found a southwest-northeast trending zone of water-level decline between Gierin Creek and Cassalery Creek with values ranging from -3 to -10 feet (Figure 17) (Thomas et al, 1999). USGS monitoring wells did not exist at the Port Williams wellfield, but the closest wells show declines on the order of -3 feet. While the USGS do not cite causes for the measured declines, reductions in recharge associated with changing irrigation practices and increased pumping associated with local population growth are likely candidates. Rainfall trends are not a likely candidate due to their relatively short-term nature. Although the USGS incorporated few monitoring wells from the Middle Aquifer in their analysis, these wells showed very similar long-term declines to those observed in nearby Shallow Aquifer wells.

While the magnitude of groundwater level declines is unlikely to significantly affect water availability in wells completed in the Lower Aquifer, wells completed in the Shallow Aquifer may be impacted. If drilled with little water column "reserve" above the well intake or maximum pump setting, 10 feet of groundwater level decline could limit pumping yield from shallow domestic wells. The regulatory issues associated with potential impacts to shallow wells in the Shallow Aquifer are complex and beyond the scope of this monitoring report. Similar water-level declines in Middle Aquifer wells could reduce yields in wells where most of the water column is already used during pumping drawdown. While domestic users are unlikely to drawdown the entire water column during pumping, multi-household water systems may make use of most (or all) of their available drawdown. The degree of yield reduction associated with the observed water-level decline depends on many factors, including pre-existing available drawdown and aquifer properties (which vary by location) and well efficiency (which varies by well). If groundwater level declines continue at the rates shown on Figure 15, maximum production capacities from wells would be further reduced.

Determination of the relative roles of the factors causing groundwater declines in the three major aquifers requires complex analysis, and is beyond the scope of this monitoring review. Previous groundwater flow modeling has shown that changes in recharge from irrigation ditches can have substantial impacts on groundwater levels in the Shallow Aquifer and in underlying aquifers (Drost, 1983 and Montgomery Water Group, 1998)⁸ . The interrelationship between water-level declines caused by Port Williams pumping, pumping by other public and private water systems, pumping by domestic wells, and changes in groundwater recharge are best evaluated by refining existing groundwater flow models for the Sequim-Dungeness Peninsula and simulating reasonable estimates of the associated changes to the groundwater budget.

8.2 Groundwater Level Trends in the Silberhorn Wellfield Vicinity
Groundwater level declines in the vicinity of the Silberhorn Wellfield were identified as problematic in the early 1990's. In 1996, PGG evaluated pumping and water-level data in the Silberhorn vicinity and reached the following conclusions:

• From the time of well installation to 1996, depth to water dropped about 17 feet over 21 years in Well 1, 13 feet over 11 years in Well 2, and 10 feet over 11 years in Well 3.
• Groundwater levels remained relatively stable between 1993-1996, although seasonal variations are noted. Deepening of private wells was prevalent between 1991-1993, but slowed down significantly between 1994-1996.

In 1996, the City expanded their water-level monitoring at the wellfield from onsite Wells 1, 2, and 3 to include 6 neighboring private wells (PD-1 through PD-6 on Table 1 and Figure 10). In addition, the City reduced their pumping rates from the Silberhorn Wellfield. Pumping records prior to 1992 do not distinguish between Silberhorn and Infiltration Gallery withdrawals, but do indicate increasing combined withdrawals over the period of record. Pumping from the Silberhorn Wellfield likely reached maximum withdrawals between 1992 and 1995.

Figure 18 presents a comparison of groundwater level trends to pumping at the Silberhorn Wellfield and Sequim precipitation. All private wells monitored by the City are located east of the Dungeness River within 0.5 miles of the wellfield and are completed in the Shallow Aquifer. Also shown on the figure is a hydrograph of Ecology Well EW-10, located across the river about 0.7 miles west of the wellfield and completed in the Shallow Aquifer⁹ . Groundwater trends in wells monitored by the City are generally similar, with some exceptions noted in the magnitude of seasonal water-level variation. Well 1 on the Silberhorn Wellfield shows a fairly stable trend between 1993-1997, followed by an accelerating rate of decline from 1998-2001 with a significant seasonal high in the summer of 2001. The 2001 seasonal high corresponds with the irrigation season, and is likely associated with recharge from irrigation practices. Prior years also show seasonal highs during the irrigation season (July, August) or toward the season's end (September, October). Some years also show high water levels in the middle of winter (January-March), which are likely associated with precipitation recharge. The other wells monitored east of the Dungeness River show similar seasonal patterns, with even more prominent seasonal highs during the irrigation season (typically reaching maxima in September). These responses indicate that irrigation provides significant recharge to the Shallow Aquifer east of the Dungeness River.

West of the Dungeness River, Well EW-10 shows a more gradual rate of water-level decline than observed to the east and does not show a similar magnitude of seasonal variations. The scarcity of data from this well prevents comprehensive evaluation of seasonal trends, however less variation can be discerned. The Dungeness River may form a partial "hydraulic boundary" that separates Shallow Aquifer responses to either side; however, insufficient information were unavailable to discern the hydraulic connection between the aquifer and the river at this locationr¹⁰ . While rainfall is likely the same on both sides of the river, irrigation practices and ditch seepage losses may vary significantly.

The declining water-level trend near the Silberhorn Wellfield over the past five years does not appear to be related to Silberhorn pumping. In general, withdrawals from the wellfield have reduced from 1995 through 2001, yet groundwater levels also declined. Local groundwater level trends do not appear to be particularly sensitivity to low pumping years (1996, 1999, 2001) or high pumping years (1997, 2000). Other likely causes for the observed trends include changes in precipitation, irrigation recharge, and construction of new domestic wells. While consecutive low-rainfall years in 2000 and 2001 may have affected the tail end of the observed decline, variations in precipitation cannot explain the declining trend noted over the last 10 to 15 years. Depth to water in Well 1 was 39 feet when installed in 1975, and has recently fallen to below 65 feet. The groundwater level decline that caused local residents to deepen their wells in the early 1990's has continued despite significant reductions in Silberhorn pumping. More detailed analysis of local changes in irrigation recharge, other pumping withdrawals, and local hydrogeologic constraints would be required to better understand the cause(s) of the observed declines.

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⁶Because local pumping rates are relatively high and vary both daily (between day and night) and seasonally, true static conditions are not expected near the wellfield. However, trends in minimum depth-to-water are generally representative of aquifer trends with the immediate effects of pumping drawdowns removed.   Return

⁷A hydrograph for Well PD-8 is not shown on Figure 15 because it almost exactly matches the hydrograph for Well PD-7. Although Well PD-8 was drilled to a depth of 247 feet, it is completed in the Shallow Aquifer at a depth of between 40 and 52 feet below land surface.   Return

⁸Both studies compared irrigation-caused groundwater level declines in the Shallow and Middle Aquifers, and estimated declines in the Middle Aquifer to be smaller than those in the Shallow Aquifer. Newly available hydrogeologic data (Thomas et al, 1999) will allow refinement of the models for improved evaluation of water-level impacts.   Return

⁹Well EW-10 has a similar groundwater elevation to Well 1 on the Silberhorn Wellfield. Its depth-to water is significantly greater because the land surface elevation is 60 feet higher at EW-10.   Return

¹⁰While the groundwater elevation in Silberhorn Well 1 averages about 250 feet msl, the Dungeness River is mapped at an elevation of 260-270 feet msl. However, groundwater levels in the Shallow Aquifer may be higher immediately below the river due to: 1) recharge from streambed seepage and 2) downward gradients from the water table to the deeper zone in which Well 1 is completed.   Return