Development of a Conceptual Site Model for Evaluating Seawater Intrusion under Sea Level Rise Scenarios Using Analytical Methods

Island County is located in the Puget Sound of Washington State and includes several islands, the largest of which is Whidbey Island. Central Whidbey Island was chosen as the project site, as residents use groundwater for their water supply and seawater intrusion near the coast is known to contaminate this resource. In 1989, Island County adopted a Saltwater Intrusion Policy and used chloride concentrations in existing wells in order to define and map “risk zones.” In 2005, this method of defining vulnerability was updated with the use of water level elevations in conjunction with chloride concentrations. The result of this work was a revised map of seawater intrusion vulnerability that is currently in use by Island County. This groundwater management strategy is defined as trigger-level management and is largely a reactive tool. In order to evaluate trends in the hydrogeologic processes at the site, including seawater intrusion under sea level rise scenarios, this report presents a workflow where groundwater flow and discharge to the sea are quantified using a revised conceptual site model. The revised conceptual site model used several simplifying assumptions that allow for first-order quantitative predictions of seawater intrusion using analytical methods. Data from water well reports included lithologic and well construction information, static water levels, and aquifer tests for specific capacity. Results from specific capacity tests define the relationship between discharge and drawdown and were input for a modified Theis equation to solve for transmissivity (Arihood, 2009). Components of the conceptual site model were created in ArcGIS and included interpolation of water level elevation, creation of groundwater basins, and the calculation of net recharge and groundwater discharge for each basin. The revised conceptual site model was then used to hypothesize regarding hydrogeologic processes based on observed trends in groundwater flow. Hypotheses used to explain a reduction in aquifer thickness and hydraulic gradient were: (1) A large increase in transmissivity occurring near the coast. (2) The reduced aquifer thickness and hydraulic gradient were the result of seawater intrusion. (3) Data used to create the conceptual site model were insufficient to resolve trends in groundwater flow. For Hypothesis 2, analytical solutions for groundwater flow under Dupuit assumptions were applied in order to evaluate seawater intrusion under projected sea level rise scenarios. Results indicated that a rise in sea level has little impact on the position of a saltwater wedge; however, a reduction in recharge has significant consequences. Future work should evaluate groundwater flow using an expanded monitoring well network and aquifer recharge should be promoted by reducing surface water runoff.

Evaluation of the Relationship Between Saturated Hydraulic Conductivity and Grain-Size Distribution of Fluvio-Glacial Deposits, Puget Lowland, Washington

The relationship between saturated hydraulic conductivity (Ks) and grain-size distribution was evaluated for 49 sites underlain by either glacially over consolidated or normally consolidated fluvio-glacial deposits in the Puget Lowland. A linear regression comprising pairs of grain-size analyses and pilot infiltration tests predicts Ks with a 1 sigma uncertainty of a factor of about 3.5 with 70% of the population variance accounted for. The correlation coefficient R^2 of about 0.90 shows that there is a strong correlation between the grain-size distribution and Ks. In contrast, a widely applied analysis proposed by Massmann (2003) explains only 20% of the population variance for normally consolidated materials with an R^2 of only 0.15. That analysis entirely fails to explain the population variance for over consolidated materials. The method developed in this study is recommended for determination of Ks for fluvio-glacial deposits of the Puget Lowland.

Physical Characteristics and Estimated Hydraulic Conductivity from Grain-Size Distribution for Vashon Till, South-Central Puget Lowland, Washington

Population growth, urban development, and increased commercial and industrial activity in the south-central Puget Lowlands of Washington State has led to an increased demand for groundwater. The Vashon till is a glacially consolidated, low-permeability unit comprising unstratified clay, silt, cobbles and boulders with ubiquitous coarse-grained lenses and is an extensive surficial unit throughout the south-central Puget Lowland. Thus, understanding the physical and hydrological characteristics – specifically, the hydraulic conductivity – of this unit is a necessary component of a groundwater model. This study provides (1) a record of the physical characteristics of Vashon till deposits within the study area; and (2) an estimate of the highest, lowest, and average value of saturated hydraulic conductivity based on the grain-size distribution of Vashon till samples collected from six field sites in the Puyallup River Watershed. Analysis shows that the average moisture content ranges between about 1 and 6%, average dry bulk density is about 2.20 g/cm3, and average porosity is about 17%. Grain-size distributions show that half of the samples analyzed are well graded, while the other half is poorly graded. Grain-size distributions also show an average d10 value of about 0.20 mm, and average ff values ≤ 16%, which are key values in estimating the saturated hydraulic conductivity of over-consolidated glacial deposits. Based on these observed values, the estimates of hydraulic conductivity range from a minimum of 0.02 m/d to a maximum of 1.38 m/d in within the general Vashon till.

Computing Hydraulic Conductivity from Borehole Infiltration Tests and Grain-Size Distribution in Advance Glacial Outwash Deposits, Puget Lowland, Washington

This study evaluates two methods for estimating a soilís hydraulic conductivity: in-situ infiltration tests and grain-size analyses. There are numerous formulas in the literature that relate hydraulic conductivity to various parameters of the infiltrating medium, but studies have shown that these formulas do not perform well when applied to depositional environments that differ from those used to derive the formulas. Thus, there exists a need to specialize infiltration tests and related grain-size analyses for the Vashon advance outwash in the Puget Lowland. I evaluated 134 infiltration tests and 119 soil samples to find a correlation between grain-size parameters and hydraulic conductivity. This work shows that a constant-head borehole infiltration test that accounts for capillarity with alpha approximately 5m^-1 is an effective method for calculating hydraulic conductivity from our flow tests. Then, by conducting grain-size analysis and applying a multiple linear regression, I show that the hydraulic conductivity can also be estimated by log(K) = 1.906 + 0.102D_10 + 0.039D_60 - 0.034D_90 - 7.952F_fines. This result predicts the infiltration rate with a 95% confidence interval of 20 ft/day. The results of study are for application in the Puget Lowland.

Analysis of the Hydrogeological Conditions and the Effects of Development on the Hydraulic Head at Redmond Ridge East

Redmond Ridge East (RRE) is a large-scale master plan community in East King County, WA. In this report, I evaluate the spatial variability of the Quaternary Advance Outwash (Qva) at RRE and the time-series data for 16 water wells with the intent to better understand groundwater below the RRE area. I investigate changes between pre- and post-development conditions through the determination of temporal changes in annual water level, annual water level fluctuations, hydraulic head response to precipitation, and ambient drainage of the aquifer. I also perform a basic analysis of the annual aquifer recharge and a determination for the storage through the implementation of the water table fluctuation (WTF) method. Associated Earth Sciences (AESI) was tasked with monitoring the geological and environmental impacts during the development of RRE and collected the data I use in this report. AESI involvement in monitoring began in 1998 and extends to the present. Sixteen wells were identified in the RRE area with adequate temporal data to conduct the analysis. A comparison of the well logs and aquifer testing data allowed local variations in the Qva to be mapped. The WTF was used to determine a range of reasonable specific yield values for locations where the Qva was unconfined. Yearly average of the seasonal water level high and lows, and the fluctuations were quantified. Temporal relationships were established through linear regression. The average water level was found to be increasing in some locations, and the corresponding fluctuations were found to decrease. However, no clear change between pre- and post-development was observed. The response of hydraulic head to precipitation was investigated through an analysis of hydrographs for ten wells. Periods of consistent response and the corresponding precipitation during each period were delineated. A linear relationship between precipitation and water level change was determined. The threshold precipitation under which there is a positive response in the hydraulic head was established. No observable changes were apparent between pre- and post-development conditions. The ambient drainage for the Qva was calculated using recessional periods on the hydrograph. The transmissivity of Qva varies with thickness of the overlying lodgment till and thickness of the Qva, itself. Water level fluctuations observed in the Qva are consistent with regional observations. Localized areas in the Qva display the large 10 foot fluctuations and these anomalies are likely due to a combination of the local variability in the storativity as well as the concentration and channeling of water due to geographical variations in the Qva and the overlying topography. All trends seen in the RRE area remained relatively constant through time. There was no evidence showing an effect of development on the hydraulic head at RRE. This implies that the style and distribution of infiltration has not changed as a result of development, and that any measures in place are properly mitigating the effects of development on the RRE region.

Hydrohalite in cold sea ice: Laboratory observations of single crystals, surface accumulations, and migration rates under a temperature gradient, with application to “Snowball Earth"

When NaCl precipitates out of a saturated solution, it forms anhydrous crystals of halite at temperatures above +0.11?C, but at temperatures below this threshold it instead precipitates as the dihydrate ‘‘hydrohalite,’’ NaCl * 2H2O. When sea ice is cooled, hydrohalite begins to precipitate within brine inclusions at about -23C. In this work, hydrohalite crystals are examined in laboratory experiments: their formation, their shape, and their response to warming and desiccation. Sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite, which has the character of a fine powder. The precipitation of hydrohalite in brine inclusions raises the albedo of sea ice, and the subsequent formation of a surface accumulation further raises the albedo. Although these processes have limited climatic importance on the modern Earth, they would have been important in determining the surface types present in regions of net sublimation on the tropical ocean in the cold phase of a Snowball Earth event. However, brine inclusions in sea ice migrate downward to warmer ice, so whether salt can accumulate on the surface depends on the relative rates of sublimation and migration. The migration rates are measured in a laboratory experiment at temperatures from -2C to -32C; the migration appears to be too slow to prevent formation of a salt crust on Snowball Earth.