Date of Award

1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Civil and Environmental Engineering

Department

Civil and Environmental Engineering

First Advisor

Raymond M. Wright

Abstract

Wet weather water quality of the Blackstone River was analyzed. Three wet weather events were successfully captured for the wet weather program: Storm 1 (September 22-24, 1992), Storm 2 (November 2-5, 1992) and Storm 3 (October 12-14, 1993). Grab samples were collected at specific time intervals throughout the Blackstone River watershed. The concentrations and EMC's (event mean concentration) for different constituents were determined. Two types of violations were compared for fecal coliform in the Blackstone River: log mean > 200 md/100 ml and 10% of samples exceeding 400 md/100 ml. Acute and chronic criteria were compared with the concentrations measured for trace metals to determine violations. These criteria are used to protect the public health and the environment.

Concentration data and flow data were used to calculate mass loadings. EMCs and average flows were used to determine the value of EMM (event mean mass) at a particular station. A comparison of wet and total loadings were provided and tributary \ wet load rankings were determined. Net gains and losses per reach were calculated and major point sources and non point sources were compared.

Net pollutant changes in a reach help to identify locations of major pollutant sources. The results of this evaluation also provided insight into the relative importance of each reach through a system ranking. The ranking for each storm was calculated for both wet and total loads. The ranking, without point sources for the wet load, were also calculated for each storm. A comparison of wet and dry weather sources for different constituents was provided.

A procedure was developed to separate runoff and resuspension. This procedure was demonstrated for the reach between BWW07 and BWW08 (Rice City Pond). This reach appears to be a major source of resuspension. An estimate of annual loadings at the MA/RI state line and end of river (to Narragansett Bay) was provided. The annual loads were divided into two parts: contribution by dry weather and contribution by wet weather. Total loadings for a particular year were determined by adding wet loads with dry loads for that year.

The analysis of the concentration data showed that wet weather did impact the water quality of the Blackstone River. For UBWPAD there was no violation of maximum ammonia discharge during dry weather conditions but violations did occur in two out of three storms during wet weather (Storm 1 and Storm 3). During peak flow fecal coliforms from Worcester passed by UBWPAD without instream disinfection. This was due to the higher flows at the WWTF and the lower chlorine residual concentrations. Significantly more stations had fecal coliform violations under wet weather than dry weather.

Wet weather caused acute criteria violations. During the height of the storm, instream hardness drops resulting in lower acute criteria concentrations. The more stringent criteria typically coincided with maximum instream concentrations. The result is the possibility of short term violations. The cause of higher metal concentration may be resuspension of the bottom sediments due to high flow and velocity, runoff, or poor performance of treatment facilities subjected to increased flow during the storm.

Pollutants associated with wet weather may come from either new sources (runoff induced) or old sources (river sediments). It is important to note that the former may be easier to control and regulate than the later. The data indicate clearly that with only minor exceptions more wet load entered the river during these periods than dry load.

The information collected during the wet weather sampling program provided insights into the behavior of the sources during varying storm conditions. A relationship was developed between rainfall and wet loadings using the data collected during the three storms and previous wet weather data available for the state line and end of river. Regression plots were done for trace metals, nutrients and TSS. These equations were used to estimate the annual wet loading rates for the Blackstone River.

The dry weather data was sufficient to permit the calibration and verification of a model to describe trace metal transport as well as a model for dissolved oxygen. The dry weather models were used to estimate baseline mass loadings under steady state flow. The relationships were developed and used to estimate the annual dry weather contributions at MA/RI state line (BWW13) and end of the river (BWW21). These dry weather loadings can be compared with the annual wet weather loadings. In the long run this is exactly what is needed to allow decisions into pollutant control.

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