Modeling nutrient dynamics under critical flow conditions in three tributaries of St. Louis Bay.

Previous research results indicated that dry weather condition has complicated impacts on nitrogen dynamics; monitored and modeling data showed both increased and decreased levels. In order to facilitate the total maximum daily loads (TMDLs) development at three tributaries of St. Louis Bay estuary, the nitrogen dynamics were investigated for two designed critical flow conditions by integrating Hydrological Simulation Program Fortran (HSPF), Environmental Fluid Dynamics Code (EFDC), and Water Quality Analysis Simulation Program (WASP). The total amount of precipitation during the dry year corresponded to a flow condition with return period of 50 years, and 10-year return period for wet year. The dry year contributed more total nitrogen (TN) loads per unit flow volume. At the upstream tributaries, the computed peak reach-averaged TN concentrations were significantly higher for dry weather simulation than wet conditions, whereas at the near-bay tributary, there were no significant differences in the peak TN concentrations. Hence, for the upstream tributaries, the nitrogen TMDL calculation should be based on dry weather condition since the decision-makers are more concerned about the worse scenario.

Application and evaluation of two nutrient algorithms of Hydrological Simulation Program Fortran in Wolf River watershed.

This study performs a comparison of two nutrient algorithms of Hydrological Simulation Program Fortran, PQUAL/IQUAL and AGCHEM. Watershed nutrient models with, PQUAL/IQUAL and AGCHEM, were developed and calibrated separately with observed data in the Wolf River watershed. Compared to AGCHEM modules, the PQUAL/IQUAL algorithm was found to have several disadvantages. Examples are: (i) it is a simple loading estimation algorithm, and cannot represent the soil nutrient processes; and (ii) the interactions of modeled nutrient species in the soil cannot be simulated. The AGCHEM modules are capable of explicitly representing the comprehensive nutrient processes in the soil such as fertilization, atmospheric deposition, manure application, plant uptake process, and the transformation processes. Therefore, AGCHEM modules afford the ability to evaluate the alternative management practice and model the interactions between nutrient species. However, our modeling results indicated that the inclusion of AGCHEM modules do not significantly improve the nutrient modeling performance but rather take much more time in model development. The nutrient algorithms selection for total maximum daily loads development depends on the data availability, required modeling accuracy, and available time for model development.

Assessment of water quality conditions in the St. Louis Bay watershed.

The water quality data from 14 sampling stations in the St. Louis Bay watershed were analyzed to evaluate the water quality conditions. The differences in water quality parameters between base and storm flow events were compared to identify the pollutant sources. The results indicated that fecal coliform was the primary cause for water quality impairment of the study area. The overall water quality conditions were good in terms of dissolved oxygen, eutrophication, and total suspended solid (TSS). The dominant sources of bio-chemical oxygen demand (BOD) could be from the failing septic system; the majority of the water samples exceeding Mississippi Department of Environmental Quality (MDEQ) target levels were from base flow events. Different from BOD, the majority of the water samples exceeding the water quality criteria and MDEQ target levels were from the storm events for fecal coliform, chemical oxygen demand, total organic carbon, TKN, NO(3), NH(3), chlorophyll a, and TSS. Based on cluster analysis, the sampling stations were classified into two major categories: upstream and near-coast stations. The major differences between upstream and near-coast stations are elevation, soil texture, and impacts of human activity. The results from this research would provide useful information for total maximum daily load calculation, development of a computational watershed model, and development of best management practices for the St. Louis Bay watershed and similar study area.

Watershed modeling of dissolved oxygen and biochemical oxygen demand using a hydrological simulation Fortran program.

Several inland water bodies in the St. Louis Bay watershed have been identified as being potentially impaired due to low level of dissolved oxygen (DO). In order to calculate the total maximum daily loads (TMDL), a standard watershed model supported by U.S. Environmental Protection Agency, Hydrological Simulation Program Fortran (HSPF), was used to simulate water temperature, DO, and bio-chemical oxygen demand (BOD). Both point and non-point sources of BOD were included in watershed modeling. The developed model was calibrated at two time periods: 1978 to 1986 and 2000 to 2001 with simulated DO closely matched the observed data and captured the seasonal variations. The model represented the general trend and average condition of observed BOD. Water temperature and BOD decay are the major factors that affect DO simulation, whereas nutrient processes, including nitrification, denitrification, and phytoplankton cycle, have slight impacts. The calibrated water quality model provides a representative linkage between the sources of BOD and in-stream DO\BOD concentrations. The developed input parameters in this research could be extended to similar coastal watersheds for TMDL determination and Best Management Practice (BMP) evaluation.