14

14.0 Fecal contamination of tidal creek sediments – links to sediment phosphorus?

Lawrence B. Cahoon and Byron Toothman
Dept. of Biological Sciences
UNC Wilmington
910-962-3706, Cahoon@uncw.edu

Fecal contamination of coastal waters is one of the most serious and well known forms of pollution in our region, mandating closure of large areas to shellfishing and creating a potential human health threat. In addition to shellfishing closures in estuarine waters mandated by the N.C. Division of Shellfish Sanitation’s routine sampling, surveys of tributaries to New Hanover County’s tidal creeks show fecal contamination levels, expressed as counts of fecal coliform bacteria (Colony Forming Units (CFU)/100 ml) that often exceed designated use standards (Mallin et al., 2002). Much of the developed area of New Hanover County is served by a central sewage system that has been relatively well tested and refined since its installation, so animal wastes in storm water runoff are probably the most common cause of fecal contamination in tidal creek waters. However, fecal coliform contamination even in the absence of storm events and their immediate runoff argues for persistence of these bacteria in tidal creek ecosystems.

Studies of fecal coliform bacteria in coastal ecosystems have shown that levels of these indicator bacteria in sediments may reach very high concentrations, and are apparently maintained by favorable conditions (Rittenberg et al., 1958; Dale, 1974; Hood and Ness, 1982; Chamroux and Guichaou, 1987; Davies et al., 1995). Our preliminary data from the Bradley Creek drainage showed fecal coliform levels on the order of 10⁶ CFU m^-2 of sediment. These observations suggested that fecal coliform bacteria may have a natural refuge in tidal creek sediments, where they are shielded from harmful solar radiation, obtain needed nutrients, and find surfaces on which to attach and survive or even grow (Dale, 1974; Tate, 1978; Henis, 1987). Furthermore, even minor sediment disturbance may suspend sufficient numbers of sediment-associated fecal coliforms to cause non-attainment of use standards, even if no “new” fecal coliforms have been washed into the system (Doyle, 1985; Gary and Adams, 1985; Seyfried and Harris, 1986; Palmer, 1988; Struck, 1988; Pettibone et al., 1996). Clearly, if such a source of fecal bacterial contamination is prevalent in our coastal ecosystems, restoration of full use of these waters will be very difficult.

Recent research has shown that not all estuarine sediments support dense populations of fecal coliform bacteria, so some factor(s) in addition to recruitment must act to control the actual fecal coliform content of estuarine sediments (Dale, 1974; Hood and Ness, 1982; Chamroux and Guichaou, 1987; Davies et al., 1995). Rowland (2002) found that sediment phosphate levels were important in controlling fecal coliform bacteria survival and growth in estuarine sediments. Phosphate loading to estuarine waters is driven by storm water runoff and other sources that covary with fecal coliform loading. Cahoon (2002) showed that residential use of phosphate-containing fertilizers was a major source of phosphate to sediments in tributaries in the Bradley Creek watershed. Consequently, fecal coliform contamination of tidal creeks in New Hanover County may be driven by a complex relationship between storm water runoff, animal sources of fecal matter, and phosphate (and other nutrients) from fertilizers, all associated with residential land uses.

Previous studies of these problems supported by the New Hanover County Tidal Creeks Program have shown that sediment fecal coliforms were present at concentrations sufficient to drive closures to shellfishing and even swimming activities if suspension into a water column 1 m deep of the observed coliforms occurred. (Fig. 1).

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Figure 1. Plot of concentration of fecal coliforms if sediment coliforms were suspended in a 1 m deep water column vs. sampling date (June 21 2001 to January 19 2002). Lines at concentrations = 200 and 14 CFU/100 ml denote NC standards for human contact and shellfishing, respectively.

We continued the investigation of linkages between sediment phosphorus and sediment fecal coliform bacteria levels in New Hanover County’s tidal creeks. The issues posed by these linkages obviously have larger implications and pose broader questions than can be addressed by a relatively limited project, but one significant and appropriate question has been addressed in this study: Do sediment phosphorus levels show any correlation with sediment fecal coliform levels?

Sampling sites were located in the Bradley Creek drainage, using locations previously sampled so as to maintain continuity (Fig. 2). These locations were sampled at least monthly for sediment phosphorus, sediment fecal coliforms, temperature and salinity. The top 2.0 centimeters of estuarine sediments were cored at each site. Three sediment cores were taken randomly at each site using sterile 2.20 cm ID acrylic tubing for sediment fecal coliform analyses. Following methods developed by Rowland (2002), each sample was transferred to a previously weighed, sterile 50ml polypropylene centrifuge tube and placed on ice. The three samples were each mixed with 1L of

Figure 2. Map showing sampling locations in the Bradley Creek watershed, named for nearby streets or tributaries. A=Andover (BC-SBU), CR=Clear Run (BC-CA), E=Eastwood (BC-NBU), M=Mallard (BC-CR), S=Softwind (BC-SB), W=Wrightsville (BC-NB). North is up. See also Fig. 7.1.

sterile phosphate-buffered rinse water inside a sterile 1L flask with a stir bar. Each sample was gently stirred for 2 minutes prior to performing the membrane filtration technique. From the mixture of sterile phosphate-buffered rinse water and sediment, three 10 ml and three 1 ml samples were used for fecal coliform analysis using standard methods for membrane filtration of fecal coliform bacteria, method 9.222 (APHA, 1998). The sediment and rinse water solution were mixed before each sample withdrawal to reduce fecal coliform burial and homogenize the bacteria suspension. All plates were incubated in a water bath for 24 hours at 44.5° C. After the 24-hour incubation period, each plate was inspected for dark blue colonies. Each dark blue colony represented one colony-forming unit (CFU). Counts from each 10ml sample from each of the three cores from each site were averaged and expressed as the number of colony forming units per square centimeter (CFU cm^-2) + one std. dev.

Sediment phosphate was analyzed on a second triplicate set of sediment cores taken randomly at each sampling site simultaneously with fecal coliform samples. Sub-samples of dried sediments were weighed and analyzed for phosphate content following digestion with the persulfate-boric acid method of Valderrama (1981). This method oxidizes relatively labile forms of phosphorus to orthophosphate, and likely represents reasonably accurately the bio-available phosphorus in sediments, in contrast to more robust extraction and digestion methods that likely quantify additional phosphorus that may be less bio-available. Sediment phosphate content was expressed as ug P (g sediment)^-1.

The concentrations of fecal coliform bacteria in sediments of Bradley Creek were highly variable, ranging from values of 0 to over 3,000 CFU cm^-2 in a total of 65 samples collected between January and November, 2003. The arithmetic mean value observed was 265 CFU cm^-2 overall, which corresponds to a value of 265 CFU per 100 mls if all these bacteria were suspended in a water column 1 meter deep, a value high enough to close the water to human body contact. The standard for shellfishing is much lower, 14 CFU per 100 mls; 49 of the 65 samples exceeded this value using analogous assumptions. Mean values for the respective sampling sites were similarly higher than the human body contact standard, except for the site at Clear Run Branch (BC-CR, “CR” in Fig. 2) (Table 1). Thus, the levels of fecal coliform bacteria measured in Bradley Creek’s sediments frequently represent serious potential problems for human uses of these waters.

Table 1. Concentrations of fecal coliform bacteria in sediments at sampling sites within the Bradley Creek drainage, CFU cm^-2. Site designations as in Fig. 2.

There was no clear relationship between sediment fecal coliform bacteria concentrations and sediment phosphorus levels (Fig. 3). Occasional high values of sediment fecal coliforms occurred across most of the concentration range for sediment phosphorus, as did low values of sediment fecal coliforms. If there is a relationship between the two parameters, it may require a much larger data set and/or more elaborate analysis of multiple variables to detect it.

Fig. 3. Bradley Creek sediment fecal coliforms vs. sediment phosphorus concentrations.

Although an effect of salinity, either as a stressor for fecal bacteria or as a proxy for distance or transit time from their warm-blooded host sources, was expected, no clear relationship was detected between these two parameters, either (Fig. 4).

Again, additional data and more sophisticated analysis would help resolve any relationship if one exists.

Temperature appeared to have had an effect on the concentrations of sediment fecal bacteria. Fecal bacteria concentrations were lowest at low temperatures and generally highest at intermediate temperatures. Very low temperatures are known to limit growth rates of fecal coliform bacteria, which grow optimally at the body temperatures of warm-blooded host organisms. High temperatures may be stressful in the absence of sufficient nutrients and organic substrates.

Additional data and analysis will be required to evaluate interactions of the factors considered above in controlling the concentrations of sediment fecal coliform bacteria. It is also likely that other parameters, such as the availability of labile organic substrates, may be important.

Sediments in the Bradley Creek drainage frequently harbored significant populations of fecal coliform bacteria, particularly during the warmer times of the year when children are most likely to play in these waters. As other studies have shown that fecal coliform bacteria concentrations in sediments do indicate the presence of other fecal pathogens (Rittenberg et al., 1958; Lipp et al., 2001), it is important to consider the public health risk associated with this poorly known reservoir of contaminants. Many water-borne diseases are not properly tracked to their sources, so a significant problem may be occurring without real awareness of its cause.

Given the attributes of the Bradley Creek watershed, it is likely that animals, both wild and domestic, were the most important fecal contamination sources. One conclusion, therefore, is that pet waste management should be addressed for all residential areas in coastal watersheds, not just beach communities. Moreover, a significant population of “wildlife” that actually associates with human communities, eating human garbage and unsecured pet foods, such as raccoons and opossums, likely lives in this watershed and contributes to the fecal contamination problem. Educational efforts can reduce this problem as well. It is important to note that animal wastes can be as dangerous a source of pathogens to humans as human waste, particularly because some of the animal-derived pathogens, such as infectious protozoans, can cause infections that are difficult to diagnose and treat. For example, an AP story published recently in the Wilmington Star-News discussed how infections from a widely distributed pathogenic amoeba, Naegleria fowleri, have caused fatal brain inflammations in swimmers.

Data generated by this effort and the previous study (Rowland 2002), which provided the data in Fig. 1, have also supported a successful proposal to UNC Sea Grant for a much more thorough study of the main hypothesis and other issues addressed in this study. This research project will begin in February, 2004, and will include an effort to examine a much larger suite of parameters that may affect fecal coliform concentrations in tidal creek sediments.

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