Assessing monitoring and modeling approaches to improve water quality in the Hickory Grove Lake

摘要

Surface water quality regulated by agricultural pollution remains to be an important environmental concern around the world. Major contaminants from agriculture systems such as bacteria, sediment, and nutrients (nitrogen and phosphorus) continue to affect the designated use of a waterbody. As per the Clean Water Act legislation, water quality impairments must be addressed through the Total Maximum Daily Load (TMDL) approach. The TMDL program is a comprehensive and watershed-scale approach involving contaminant source identification and quantification, and conservation practice recommendation to reduce contaminant transport. The overall goal of this study was to improve the TMDL development process in achieving water quality goals and restoring impaired waterbodies. Specific objectives were to: (1) identify phosphorus transport pathways during rainfall-runoff events in a tile-dominated agricultural watershed; (2) demonstrate a novel approach in setting a bacteria TMDL for an impaired waterbody and; (3) determine potential locations for conservation practice placement at the watershed-scale to maximize reduction of sediment transport. The Hickory Grove Lake located in central Iowa, a waterbody impaired due to E. coli levels at the swimming beach was the focus of this study.Phosphorus (P) transport pathways in the tile drained agricultural watershed were determined through intensive monitoring during runoff events and a chemical hydrograph separation (CHS) method. Rainfall events in Spring 2013 were monitored for flow, Dissolved Reactive Phosphorus (DRP) and Total Phosphorus (TP) concentrations at the tile outlet (TO) and subwatershed outlet (SO) in the Hickory Grove Lake Watershed (HGLW). The drainage areas of TO and SO are 879 ha and 852 ha, respectively. The discharge at TO comprises runoff from surface intakes and flow from subsurface tile-drains, whereas discharge at SO comprises flow from TO and surface runoff during runoff events. The median TP concentrations during spring runoff events in 2013 at TO and SO were 0.89 mg/L and 1.13 mg/L, respectively. The TP and DRP levels at TO and SO during low flow and high flow conditions were similar. The highest P levels at TO and SO were observed during the rising limb of the hydrograph. Surface intakes accounted for 15.2% of the total discharge at SO and 23.6% of the total discharge at TO. It was also estimated that 28.2% of the TP load at SO originated from surface intakes. Due to surface intake contribution to subsurface tile-drains, similar P concentrations were observed in TO and SO. This study improves understanding of the P dynamics and transport pathways in tile drained agricultural watersheds. Therefore, contaminant source identification and quantification during TMDL development must acknowledge the underappreciated transport pathway of P (surface intake) in tile drained watersheds.The Hickory Grove Lake beach was listed on Iowa\u27s 303d list of impaired waters due to elevated E. coli concentrations, and therefore, a novel approach was proposed to develop a bacteria TMDL. Fecal bacteria monitoring data at the Hickory Grove Lake Inlet, Lake Outlet, and Lake Beach was used to develop linear regression relationships and understand the influence of fecal bacteria sources in the watershed on the Lake Beach E. coli levels. It was determined that fecal bacteria from the HGLW had very little effect on E. coli levels at the Lake Beach, instead fecal bacteria from waterfowl were regulating the E. coli levels at the beach. Spatial monitoring of the lake suggested that E. coli levels were elevated at the Lake Beach and at other locations where geese reside year-round. A TMDL developed using a Near-Shore Beach Volume model was set at 1.8 x 1011 cfu/day for the single sample mean (SSM) target and 1.01 x 1011 cfu/day for the geometric mean target. The daily fecal bacteria load from as few as 5 resident geese were sufficient to cause E. coli levels at the Lake Beach to exceed the SSM standard. Therefore, efforts to achieve the bacteria TMDL must focus on deterring the resident geese at the lake.Conservation practice recommendation and placement to mitigate contaminant transport is the next step after TMDL development. Spatial monitoring of the Hickory Grove Lake in November 2012 indicated that the east basin of the lake is now filled with sediment. The Light Detection and Ranging (LiDAR) data and precision conservation technologies were used in this study to identify potential locations for grassed waterway (GWW) placement in the HGLW to reduce sediment transport. The compound topographic index (CTI) model supplemented with 3 m LiDAR data was used to identify GWW locations. The CTI model identified all existing GWWs and recommended new locations for GWW placement at a CTI threshold of 30. The CTI model overestimated the lengths of existing GWWs suggesting a need to further extend the GWWs. The design recommendations of the predicted GWWs suggested that the total surface area required for predicted GWWs was 29.3 ha. The results of this study imply that LiDAR derived terrain attributes can be effectively used in identifying potential locations for GWWs.The overall results of the complete study suggest that conventional TMDL development may not be appropriate for all impaired waterbodies; a novel and holistic approach is required depending on the contaminant source and its transport pathways, watershed characteristics, and hydrology of the watershed.