The Mineralogy and Strength Characteristics of Selected Glaciolacustrine Clays in the Puget Sound Region

The combination of rainy climate, glaciolacustrine clays, and steep topography of the Puget Lowland creates slope stability issues for the regional population. Several glaciolacustrine deposits of laminated silt and clay of different ages contribute to the likelihood of slope failure. The glaciolacustrine deposits are generally wet, range in thickness from absent to >30m, and consist of laminated silt and clay with sand interbeds at the tops and bottoms, sandy laminae throughout the deposits, occasional dropstones and shear zones. The glaciolacustrine deposits destabilize slopes by 1) impeding groundwater flow percolating through overlying glacial outwash sediments, 2) having sandy laminae that lower strength by increasing pore pressure during wet seasons, and 3) increasing the potential for block-style failure because of secondary groundwater pathways such as laminae and vertical fractures. Eight clay samples from six known landslide deposits were analyzed in this study for their mineralogy, clay fraction and strength characteristics. The mineralogy was determined using X-ray Diffractometry (XRD) which revealed an identical mineralogic suite among all eight samples consisting of chlorite, illite and smectite. Nonclay minerals appearing in the X-ray diffractogram include amphibole and plagioclase after removal of abundant quartz grains. Hydrometer tests yielded clay-size fraction percentages of the samples ranging from 10% to 90%, and ring shear tests showed that the angle of residual shear resistance (phi_r) ranged from 11° to 31°. Atterberg limits of the samples were found to have liquid limits ranging from 33 to 83, with plastic limits ranging from 25 to 35 and plasticity indices ranging from 6 to 48. The results of the hydrometer and residual shear strength tests suggest that phi_r varies inversely with the clay-size fraction, but that this relationship was not consistent among all eight samples. The nature of the XRD analysis only revealed the identity of the clay minerals present in the samples, and provided no quantitative information. Thus, the extent to which the mineralogy influenced the strength variability among the samples cannot be determined given that the mineral assemblages are identical. Additional samples from different locations within each deposit along with quantitative compositional analyses would be necessary to properly account for the observed strength variability.

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.

A Study of Shallow Groundwater Migration through Anthropogenic Fill along a Seawall

This report provides the findings and opinions of a historical document review, hydraulic balance calculation, and proposed additional study for a property that was historically used as a bulk petroleum storage and distribution facility. The property lies along the base, west, of a heavily vegetated bluff with a tidally influenced body of water west-adjacent to the property. The western portion of the property is bounded by a seawall spanning approximately 3,200 linear feet trending north-south. The seawall’s construction details are not known, save for a 225-foot section of driven sheet pile wall located within the northern portion of the property’s seawall. Due to the presence of petroleum hydrocarbons in soil and groundwater at the property, a cleanup action for the property will likely be overseen by the state regulatory agency. The property is currently undergoing remedial investigation in an effort to identify the lateral and vertical extent in which contaminants at the property have come to be located, also known as the “site” as defined by the Model Toxics Control Act (MTCA). The majority of the property bounded within the seawall area has been characterized; however, the shoreline sediments located immediately west-adjacent of the seawall have not been properly delineated. Identifying the bounds of the site to the west within sediment is pivotal for the purposes of the remedial investigation. Since the west-adjacent shoreline is so extensive, conducting a complete sediment sampling event along the entire shoreline would be cost-prohibitive due to analytical costs and logistical issues at the property. Because of the extensive nature of the shoreline, it would greatly benefit the client and project to focus sampling efforts at areas of greater risk for contaminants along the shoreline by identifying potential preferential pathways for contaminants to migrate off of the property and into adjacent shoreline sediments. The review of historical studies of the property yielded some useful information; however much of the findings included within the historical studies were lacking original raw data, therefore limiting the information obtained. The calculated hydraulic balance for the property yielded a relatively large surplus of recharge to groundwater after precipitation events, reinforcing the concept that contaminant have potentially historically, and currently, been migrating into the adjacent shoreline through preferential pathways along the seawall. Due to the limitation within the historical studies for the property as well as the groundwater recharge identified in the hydraulic balance, an additional study was proposed in an effort to provide additional aquifer characteristics along the seawall, and the ability to observe flow propagation at and proximate to the seawall in two-dimensions through time without the need to piece separate studies together. This proposed study includes a single simultaneous tidal study which comprises select monitoring points along the seawall. This report has identified the need for additional data that can be collected through available avenues for the property based upon the client’s desires and project needs. Ultimately, the proposed additional study is suggested based upon its relatively low capital investment, and ability meet the requirements relevant to the specific project needs and scope. Assuming preferential pathways are identified through the additional study proposed within this report, a representative and cost-effective sediment sampling plan can then be put in place in an effort to define the site.