Inter-Storm Dissolved Organic Matter Variability and its Role in Microbial Transport during Urban Runoff Events

The harvest and reuse of stormwater is a current challenge, and one major concern is associated with the wide range of pollutants present in the stormwater runoff. The pathogens and chemicals that have been indentified in stormwater runoff impair the ecological health of receiving water bodies and pose significant human health risk, particularly microbial pathogens. These microbes attach to suspended particulate matter, particularly organic material for their transport. Aryal et al. (2012) investigated the relationship between organic matter present in stormwater and two specific microbes, Escherichia coli and Enterococcus spp, showing that the increase of hydrophilic organics and humic substances—such as dissolved organic carbon, sulfur, nitrogen, and oxygen—hindered the transportation of microbial pathogens present in the stormwater runoff. –Genevieve Heger
Aryal,R., Sidhu, J.P.S., Chong, M.N., Toze, S., Keller, J. , Gernjak, W. 2012. Inter-Storm Dissolved Organic Matter Variability and its Role in Microbial Transport during Urban Runoff Events. 18

Aryal et al. studied five stormwater events in South East Queensland throughout the months of March, April, and May, 2011. Stormwater samples were collected from the Fitzgibbon drain, which is a medium density residential urban catchment. A total of four wet weather samples (31 March, 4 April, 18 April and 30 May) and one dry weather sample (7 March 2011) were collected. The dry sample was collected by a grab sampling method, whereas the wet weather samples were collected using three automatic sampler methods (ISCO 6700 series). A submersible Argonaut Flow Doppler (Thermo Fisher Sci.) was installed to measure the stormwater flow, and the auto-samplers were programmed to fill up to 24 x 20 L high density polyethylene (HDPE) container (Food & Drug approved grade) during the storm duration. The collected stormwater samples were analyzed for faecal indicator bacteria within 10 hours of collection, and a portion of each sample was stored at 4oC to be chemically analyzed within a week.
The quantification of faecal indicating bacteria (FIB) was determined by the standard membrane filtration method, where samples were filtered though nitrocellulose filters, transferred to respective selective agar plates, incubated overnight, and finally counted to calculate the average number of colony forming units (cfu 100mL-1). The quantification and identification of organic matter was analyzed using UV spectrophotometry (Varian 50 Bio) scanning between 190-400 nm, in addition to liquid size exclusion chromatography with organic carbon detection (LC-OCD, DOC Labor, Dr. Huber) to determine dissolved organic fractions.
Results indicate that E. coli and Enterococcus spp. numbers were significantly higher during the wet period than the dry period. The dry weather showed the highest absorbance indicating a higher concentration of organics in the sample The E. coli numbers varied between 102 to 103 100 cfu mL-1 during the dry and wet periods whereas, the corresponding numbers for Enterococcus spp. varied between 102 to 104 cfu 100mL-1.
The data show some differences in the absorbance between the five events, which indicates variation in the concentration of dissolved organic matter. The dry weather sample showed the highest absorbance, indicating a higher concentration of organics. It was believed that the stagnant water in the drainage seemed to helped to release/decompose organics present in the bed sediment (Spellman 2009). Other samples showed a decrease in concentration from every runoff event to the next.

There was a negative correlation between the concentration of microbes and dissolved organic carbon, especially hydrophilic organic material. Within the hydrophilic fraction, humic substances hindered the FIB transport more than the other organics. Humic substances showed a negative correlation with microbial densities indicating that their presence may hinder the transportation of microbes. While it is made clear that a decrease of organic concentration favored the increase of FIB numbers in stormwater; the study suggests that further research is necessary to understand the influence of dissolved organic matter in microbes transport via stormwater.

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