UNIQUE GEOCHEMSITRY OF GRAVELLY LAKE, TACOMA, WA: EVIDENCE FOR A DEEP GROUNDWATER COMPONENT

Gravelly Lake (GL) is one of numerous kettle lakes in Pierce County. The substrate in the area is composed of several glacial outwash and till units. This lake covers an area of ~65 hectares and has no surface water inputs; the main water source is groundwater. American Lake (AL) lies up gradient from GL suggesting transport of groundwater in the shallow aquifer from AL to GL. Compared to AL, GL exhibits more stratification, greater clarity (Secchi depths to 9.5m), and higher SiO₂, Ca, Mg, and alkalinity. Another unique feature is a color change from steel-blue to aquamarine during summer months.  The primary goals of the project are: to characterize the composition of water at different depths and seasons, establish processes responsible for chemical variation, and identify factors responsible for unique chemical characteristics of GL.     

Over an eight-month period 80 samples were collected and analyzed. Levels of major cations (Si = 5.6 ppm, Ca = 13.4 ppm, Mg = 5.2 ppm) are notably higher (up to 390% for Si) in GL than AL. GL showed more variation with depth than in AL. Month-to-month variations in GL spring chemistry track variations in AL water chemistry, indicating rapid (< 1 month) groundwater flow from AL to GL springs. However, the compositions of the groundwater springs reflect more than one end-member. Spring compositions vary (from one to another and month to month), consistent with mixing between AL water and an additional groundwater component. The elevated Si levels are the result of deep groundwater. The chemistry of GL is most similar to the deep groundwater component; whereas the springs mostly resemble AL. Seasonal trends in whole lake element loading (Si increased 2.5 times from June-Jan.) reflect changes in relative contribution of shallow versus deep groundwater. A recent USGS study has identified a localized linear disruption of the confining layer between the shallow aquifer and deeper aquifers that may provide a window for deep groundwater transport. We conclude that deep groundwater is the dominate source of water at GL during summer months when lower rainfall results in diminished flow from AL. Diatoms were also recognized in sediment collected from GL. We attribute the aquamarine color of GL water in summer months to precipitation of calcite driven by diatom photosynthesis when Si is high.