IMPACT OF STORM DRAIN RUNOFF ON WATER QUALITY OF THE MILL RIVER, NORTHAMPTON, MASSACHUSETTS

Eight storm drains discharging from the Smith College campus into the lower portion of the Mill River were sampled during a 104 mm rain event in late September 2008. Although the watershed area of the drains is very small (18 hectares) compared to the watershed of the Mill River (14,000 hectares), runoff from the drains had a measurable impact on water quality in the Mill River at the beginning of the event. Suspended sediment concentrations in the river and storm drains were measured gravimetrically after filtering through a 0.45 µm membrane filter. River samples were collected every two hours using an ISCO model 3700 autosampler. River turbidity was constantly monitored using a Campbell Scientific OBS+ optical sensor. River stage was monitored throughout the event using a pressure transducer connected to a Campbell Scientific datalogger and discharge was estimated from a stage discharge relationship. Storm drain discharge was estimated using hydraulic equations for flow through pipes and grab samples were collected periodically throughout the event.

Peak discharge in the Mill River (550 cfs) occurred 19 hours after the beginning of rainfall when suspended sediment concentrations reached 11 mg/L. Storm drains reacted almost instantaneously to precipitation rates and while maximum flows were low (2 cfs) compared to the river, suspended sediment concentrations were high (642 mg/L). Storm drain discharge occurred before there was any significant increase in river discharge and therefore had an immediate impact on sediment load. During the first 7 hours of the event the storm drains accounted for approximately 5 percent of the total sediment load of the river even though the watershed area associated with the drains represented less than 0.1 percent of the watershed. However, after this initial period the effects of the storm drains were insignificant as flow from the rest of the watershed dominated the system. Total sediment discharge during this event was approximately 15 metric tons. Storm drains draining suburban areas were found to produce far more sediment and dissolved solids than storm drains draining athletic fields.