Date of Award

Fall 2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Program/Concentration

Ocean and Earth Sciences

Committee Director

Fred C. Dobbs

Committee Member

Andrew S. Gordon

Committee Member

Alexander Bochdansky

Call Number for Print

Special Collections LD4331.O35 N24 2012

Abstract

This research seeks to evaluate the impact of an urban, multi-use neighborhood on the bacteriological quality of water in Knitting Mill Creek, a blind arm of the Lafayette River, a sub-estuary of the lower Chesapeake Bay. A principal objective was to determine the effects of rainfall, a surrogate for run-off, on water-column concentrations of fecal-indicator bacteria (FIB). I collected water samples weekly (September 2009-December 2010) and biweekly (January 2010-December 2011) at a storm-sewer outfall and a nearby marina and quantified their E. coli and enterococci concentrations using Colilert-18 and Enterolert, most-probable-number assays. In addition, I took monthly samples for these FIB at 11 stations along the creek's long axis (September 2009-December 2011).

Great variation in FIB concentrations was observed during the study; values varied over four orders of magnitude. In almost every instance, FIB concentrations were greater at the storm drain outfall than at the marina. Greater bacterial concentrations were observed in the warmer months, when the majority of samples exceeded the EPA's single sample maximum of 235 MPN/l00mL for E.coli and 104 MPN/l00mL for enterococci. Thus, Knitting Mill Creek was not suitable for swimming during a large portion of the year. Of all the environmental parameters studied (water temperature, salinity, wind speed, and time of day when samples were collected), only water temperature showed a significant correlation with FIB concentrations. There was no

significant correlation with rainfall except during periods of extended and heavy precipitation. Monthly samples along the creek's long axis consistently showed bacterial concentrations of the upstream stations were greater than those at the downstream stations. In particular, stations 1 and 2, located at the blind end of Knitting Mill Creek, were 'hot spots' for FIB.

The second part of my thesis was a comparison between the well-established Colilert-18 and the newer Coliscan method for the enumeration of E. coli. I compared the two methods using pure cultures and field samples. For within-method comparisons, a fresh water matrix yielded counts statistically higher than did a saltwater matrix. Comparing between methods, Coliscan yielded higher counts than Colilert-18 in a salt water matrix, but not in a fresh water one. A related experiment, however, indicated no difference between the two methods across a wide range of salinities. When I compared the methods using data from a year's worth of estuarine field samples, Colilert-18 yielded significantly higher estimates than Coliscan. Furthermore, over the range of E. coli values encountered, there was a low level of correspondence between the two methods. Taking all the above points into consideration, I found the two methods are not uniformly comparable. The relatively newer Coliscan method cannot be considered equal to the well-established Colilert-18 method.

Because of its ease of use and minimal requirements for equipment (notably, no incubator is necessary), the Coliscan method is becoming popular with citizen volunteer groups involved in water quality monitoring. In addition to its under-reporting relative to Colilert-18, however, there are several disadvantages to the Coliscan method. It is relatively insensitive, the gel takes two to three times longer to solidify than stated by the manufacturer, and in most cases, an incubation period of 48 hrs is required to best distinguish colonies.

There are important implications of this research with respect to public health and environmental management. This is one of the few studies which have compared the two methods, Colilert-18 and Coliscan Easygel, used for the enumeration of E.coli.

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DOI

10.25777/qes3-sz04

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