Resultados

concentrations. For ammonium-N, average spring concentrations were significantly higher than summer ammonium concentrations, probably due to cooler temperatures and less available soil air. For example, after the winter-spring wet season the watershed soil would have a higher water content, leaving less pore space available for air to oxygenate soil ammonium into nitrite or nitrate. Trojan et al. (2003) found groundwater concentrations of nitrate were 0.6 mg/L in

undeveloped areas, indicating a background nitrate-N value of 0.14 mg/L. Nitrate-N

concentrations according to Stackelberg et al. (1997) showed 0.07 mg/L in undeveloped areas, 2.6 mg/L for new urban areas, 3.5 mg/L for old urban areas, and 13 mg/L for agricultural land use. Several sites in the Carters Creek watershed during low flow had comparable background values, between 0.07 mg/L NO3-N at Burton 2 to 0.23 mg/L NO3-N at Bee Creek, indicating healthy background levels of nitrate. Carters Creek also had lower nitrate concentrations for urban areas than the average from Stackelberg et al. (1997). Slow-flowing waters with high temperatures, such as Wolfpen, Hudson, Bee and Briar Creeks from this study, are likely to have significant denitrification potential, moderating the effect of urbanization somewhat (Schaffner et al., 2009). However, Carter 4 and 5 had concentrations derived from nutrient rich WWTP effluent discharged upstream of the sampling sites that approached the high-nitrate-leaching agricultural croplands. In an undisturbed old growth forest watershed the in-stream DON:TDN ratio may be 0.60-0.95, while urban ratios are closer to 0.35 (Pellerin et al., 2006). Surprisingly, the DON:TDN ratio for most of the urban subcatchments is closer to the undisturbed bracket, perhaps because of the higher nitrogenous organic matter content in the stream. Sites

downstream of the WWTP are more consistent with the urban bracket or even lower at 0.15-0.25, due to the high nitrate content of the effluent.

4.4. Orthophosphate

In addition to nitrate, Carter 4 and 5 sites also had the largest orthophosphate concentrations in the Carters Creek basin. The sites with the next highest phosphate concentrations were those with headwaters in golf courses. According to King et al. (2007), a golf course in Austin, Texas produced 1.2 kg NO3-N ha^-1 yr^-1 and 0.51 kg PO4-P ha^-1 yr^-1, or the equivalent of 3.3% and 6.2%

applied N and P, respectively. Though the nitrate inputs from storm water did not cause

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concentrations that threaten the stream aquatic habitat, the phosphate contribution posed a threat according to the USEPA standard of 0.1 mg/L (King et al., 2007). Greater phosphate inputs to the stream were measured during fall and winter, when the turf grass metabolic rate was slowing down (King et al., 2007). The Carters Creek watershed also showed a significant phosphate contribution from subcatchments with golf courses, especially in fall and winter. Interestingly, Burton 5 also has golf course headwaters, but does not show any increase in phosphate compared to the other sample sites, possibly because all golf course runoff is diverted to a small lake so that particulates and the associated contaminants are allowed to settle out of the stream suspension.

Fertilizer application of superphosphate to pasture in New Zealand led to an accumulation of contaminant fluoride in the top 200 mm of soil, in a mobility pattern similar to that of phosphate (Loganathan et al., 2001). Triple superphosphate fertilizer has a F:PO4 ratio of 0.085, whereas single superphosphate (SSP) has a ratio of 0.20 (Loganathan et al., 2001). In this study, fluoride concentration during low flow was significantly higher in Hudson than in Briar 2, Burton 3-5 and the upper Carter sites, while Wolfpen had higher fluoride concentrations than Briar 2 and Carter 3. Both Hudson and Wolfpen had headwaters from golf courses. Annual fertilizer application to turf grass in golf courses, residential lawns and urban green spaces at the

beginning of the growing season would also explain why the spring fluoride values are higher in the watershed as a whole than in the rest of the year.

In addition to being a commonly added ingredient in toothpaste (Buzalaf et al., 2008), sodium monofluorophosphate has been used in the construction industry as a corrosion inhibitor to steel reinforcements in concrete structures for the last twenty years (Chaussadent et al., 2006).

An aqueous solution of the compound is applied to the concrete surface and diffuses into the porous concrete matrix, where it coats the steel reinforcements, reacts with Ca(OH)2 to form

insoluble apatites or hydrolyzes into phosphate and fluoride ions (Ngala et al. 2003). Phosphate and fluoride adsorb to soil particles more strongly than other anions commonly found in soil solution, greatly reducing ion mobility. In an experiment on a northern hardwood spodosol soils, Nodvin et al. (1986) reported that phosphate has a linear adsorption rate of 0.99 of initial mass in the soil solution, followed by the fluoride adsorption rate of 0.80. The strong relationship between phosphate and fluoride seen in the Carter Creek watershed may also indicate erosion of surface sediment, carried into the stream along with the adsorbed anions. Nevertheless, fluoride concentrations were maintained safely below the Secondary Maximum Contaminant Level (SMCL) of 2 mg/L for drinking water (USEPA, 2009).

In-stream orthophosphate also revealed a relationship with in-stream SAR in this study. As sodium is adsorbed onto soil cation exchange sites under high pH conditions and replaces divalent and trivalent cations, the increasing negative charge of the soil particle repels any nearby phosphate, which becomes much more soluble (Naidu and Regasamy, 1993). Once in the soil solution, phosphate is easily assimilated by plants or lost to groundwater or runoff and then the stream channel. Curtin et al. (1995) found an SAR of 20 significantly decreased the binding ability of clay minerals and greatly increased the water-extractable fraction of total phosphorus. By comparison, mean annual SAR in the Carters Creek watershed varied from 2.9 in Carter 3 to 20.3 in Wolfpen. In Results Section 3.4, orthophosphate showed a strongly significant relationship with SAR in summer, fall and winter, but not in spring. This may reflect the opposing factors of the flushing of Na-PO4 complexes during rain events and the rapid turf assimilation of solubilized phosphate when grass and other vegetation is coming out of winter dormancy (King et al., 2007).

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