Review of the St. Johns River Water Supply Impact Study by National Research Council

Author:National Research Council
Language: eng
Format: epub
Tags: Environment and Environmental Studies: Ecology and Ecosystems, Earth Sciences: Water and Hydrology
ISBN: 9780309225700
Publisher: The National Academies Press
Published: 2012-12-24T00:00:00+00:00

Overall, measured release rates for nutrients and DOC from Lake Poinsett wetland soils were low, indicating that the soils were recalcitrant to oxidation upon exposure to air. The workgroup provided supporting evidence for the low release rates in the form of carbon-tonitrogen (C:N) ratios in the soils, which they reported to be in the range 10-13. Soils collected from wetlands in segments 6 and 7 as part of the workgroup’s synoptic sampling had similar average C:N ratios. In contrast, wetland soils in some other upper basin marsh conservation areas tend to be higher (14-17). According to a soil chemistry book (Brady and Weil, 2002) cited by the workgroup, the biodegradability of soil organic matter decreases with decreasing C:N ratios. In this book, soils with C:N = 15-30 are considered to be “active,” decomposing in a timeframe of 1-2 years; those with C:N = 10-20 are considered “slow,” decomposing in 15-100 years; and those with C:N = 7-10 are considered “passive,” and are stable over timeframes of a few centuries.

Flow-weighted DOC concentrations for June-October found from the mass balances were in the range 24-30 mg/L for Lake Poinsett and 25-35 mg/L for Lake Winder. TP generally was in the range 0.10-0.20 mg/L in both lakes; higher and lower values occurred in a few years. Similarly, TKN generally was between 1 and 2 mg/L, but again higher or lower values occurred in a few years. When additional DOC and nutrients released from exposed wetland soils were taken into consideration, average concentrations of DOC in the river increased by < 0.1 to ~0.2 mg/L, depending on the value of the rate coefficient (K) used in the tanks-in-series removal model. For TP the concentrations increased by ~0.001 to ~0.01 mg/L depending on the value of K, and for TKN, concentrations increased by < 0.01 to ~0.02 mg/L. In all cases, the increases are a small fraction of the average concentrations calculated in the mass balances, and the workgroup concluded that oxidation of wetland soils would not cause significant effects on nutrient conditions in lakes of the St. Johns River.

For DOC, the workgroup conducted a further analysis to evaluate potential effects on dissolved oxygen concentrations in Lake Poinsett. Monitoring data showed that low dissolved oxygen (DO) concentrations occur in the lake during summer, and the workgroup hypothesized that the low DO was caused by oxidation of the high concentrations of DOC in the river water. A photochemical mechanism for DO depletion that involves the colored (humic) portion of DOC has been known for decades (Miles and Brezonik, 1981; Xie et al., 2004), but microbial processes also may be involved. The workgroup found a small negative correlation (r2 = 0.2) between DOC and DO concentrations in Lake Poinsett, and using a more complex multiple regression model they calculated that in-lake DO concentrations would change by negligible amounts (0.00 to ~0.02 mg/L depending on the value of K used in the tanks-in-series model).

Based on the findings summarized above, the workgroup prepared a series of summary tables for

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