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The Sheepscot River Turbidity Pilot Study, - page 10 / 12





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provides for 75 foot wide undisturbed buffers for new construction along most river banks.  Sensitive areas may have larger set-backs.  However, smaller streams only have 35 foot set-backs.  Furthermore, some logging is allowed within the shoreland zone, provided that ground cover and a mix of tree sizes are maintained.  Thus, smaller streams will be the most vulnerable to land disturbances.  

The spatial patterns of turbidity and TSS were found to be different for each rain storm.  So, there are clearly many sources of NPS pollution and their relative contribution of each source changes through the year.  This variable pattern may be a function of the spatial distribution of rainfall in the watershed, seasonal changes in land use, and the different hydrological characteristic of different seasons.  For instance, the hydrologic changes refer to changes in the relative amounts of runoff/melt water, compared to shallow sub-surface and groundwater flows (Mulholland 1993).  When the ground is saturated, water is perched above the normal groundwater level, and lateral flow over the land surface and through the upper soil is common.  Runoff flows into the river channel most quickly, followed by shallow sub-surface flows, and then groundwater.  According to this conceptual model, surface runoff dominates the first flush of water.  Soon after the peak flow, shallow sub-surface flow becomes more important.  During the last part of the recovery period and during baseflow conditions, groundwater dominates river flow.  This helps explain why NPS pollution (i.e., polluted runoff) is most abundant in the first flush of a stormwater event.

From years of experience on the Sheepscot River, we know that turbidity events are common in the late winter and spring and often last for weeks at a time.  These long-term turbidity events are associated with high flows, but continue between storm events, even when the river stage is falling.  Sometimes this high flow/turbidity is clearly due to snow melt, but we have not studied it enough to be certain this is always the case.  Because the long-term turbidity events are more than a first flush problem, there are evidently in-the-river sources of turbidity.  These might be stream bank failures or winter frost-heave of clay soils.  In our present study, the samples in May, June and October were taken in short term turbidity events associated with individual storms.  We believe that our May samples were taken too late in the year to detect the long-term events.  We recommend that automated environmental monitors (so called “data sondes”) with turbidity probes be used for documenting these long-term turbidity events.

Given the good correlation of TSS with turbidity, and as a cost-saving measure, we conclude that turbidity alone be used as an “indicator value” in future studies.  The response of the SVCA volunteers was excellent, and we feel that additional on-call event monitoring in the Sheepscot watershed is possible.  However, since the turbidity sources are always changing, we feel that watershed NPS surveys must be coupled with water quality data to identify and fix erosion sites.  NPS control efforts should be directed to Meadow Brook and Choat Brook and to mainstem sites on the middle and lower West Branch and Sheepscot River.  During a rainy day NPS survey, a turbidity tube could be used to follow turbidity problems upstream to their sources.

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