Silica (SiO2) is the most commonly found compound in earth's crust. Silica is most present in the water cycle as dissolved silica in groundwater because as water passes through the crust, weathering materials such as soil and rock, it dissolves minerals, including silica. Groundwater and, to a lesser extent, runoff are the primary sources for rivers, lakes, and streams. Dissolved silica can be depleted from these waters by biological activity, either aquatic or terrestrial. Diatoms, specifically, uptake silica to contruct thier hard parts

In the Nequasset watershed, the general trend is that during the summer and into the early fall dissolved silica remains lower (0-4 mg/l) than it does during the late fall through the early spring when it is higher (4-8 mg/l). From early May, the trend again seems to be headed downward (the data stops here). This seasonal pattern is explained by the terrestrial uptake of silica by vegetation on the banks of the water bodies[1] and by diatoms within the water itself. As the vegetation dies off in the fall, the uptake of silica decreases and the concentration of silica in the water increases due to the absence of terrestrial uptake. And as vegetation returns in the spring, silica is sequestered, causing the concentration to fall again.

Wright Lane had the highest silica concentrations throughout most of the seasonal pattern. Wright Lane is a small watershed that lacks a sizeable wetland. Wetlands uptake a significant amount of silica; further, a small watershed indicates that Wright Lane does not get significant precipitation runoff. Due to the small size of the watershed and the absence of a sizable wetland, it can be assumed that Wright Lane is fed primarily by groundwater. As mentioned above, groundwater has the highest concentrations of silica. These two affects, the lack of silica uptake by a wetland and the high silica concentration from groundwater, combine to give Wright Lane the highest silica concentration. At the other extreme, Kohler Pond dropped to concentrations of dissolved silica close to zero during the fall. Upstream from Kohler Pond is a sizable wetland, which possesses diatoms that use silica to construct their cell walls. Thus, diatoms, which are particularly prevalent in wetlands, contribute to the low silica concentration at Kohler Pond as well as at Nequasset Park for similar reasons

[1]. Fulweiler R. W, Nixon, S. Terrestrial vegetation and the seasonal cycleof dissolved silica in a southern New Englandcoastal river. Biogeochemistry (2005) (74.1)

Wright Lane tributary: generally displays highest silica readings because of few wetlands and the relative dominance of groundwater inputs within the sub-watershed.