DEEP GROUNDWATER INFLOW EXPLAINS ANOMALOUS CHEMISTRY OF A KETTLE LAKE, SOUTH PUGET SOUND, WASHINGTON

Gravelly Lake (GL) is one of numerous kettle lakes scattered across the glacial outwash plain at the south end of Puget Sound. It covers an area of ~65 ha, has an average depth of 12m (max 17m), and no surface water inputs; the main water source is groundwater. GL differs markedly from other lakes in the vicinity in having higher cation levels and alkalinity, greater clarity (Secchi depths to 9.5m), and a color change from steel-blue to aquamarine during summer months. The primary goals of the project are: (1) to characterize the composition of water at different depths over the course of a year, (2) to compare GL with American Lake (AL), which lies upgradient and is believed to supply water to GL by shallow interflow, and (3) to identify the factors responsible for the unique chemical characteristics of GL.

Analyses of 120 samples (including springs) collected at monthly intervals between 6/2008 – 5/2009 reveal that the GL water (weighted average of samples from different depths) has higher cation levels (Si = 5.6 ppm, Ca = 13.4 ppm, Mg = 5.2 ppm) than AL (Si = 1.1 ppm, Ca = 10.5 ppm, Mg = 3.8 ppm). Seasonal trends in GL were not caused solely by stratification. Monthly variations in GL spring chemistry track variations in AL water chemistry, indicating rapid groundwater flow (< 1 month) from AL to GL springs. However, the compositions of the groundwater springs require a second end-member having higher levels of Si and Na. Spring compositions vary by month, consistent with mixing between AL water and an additional groundwater component. Elevated Si levels are the result of deep groundwater. The average chemistry of GL closely resembles the deep groundwater component; whereas the springs mostly resemble AL. Seasonal trends in whole lake element loading (Si increased 1.5 times from June-Jan.) reflect changes in relative contribution of shallow versus deep groundwater. Elevated radon levels in springs (172-346 pCl/L) confirm contributions of groundwater and a recent USGS study (Dinicola, 2005) 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 dominant source of water at GL during summer months when lower rainfall results in diminished flow from AL.