Research - Geochemical Modeling and Natural Attenuation

Research - Non-point source runoff and surface water quality in urban streams

Background

As for many other urban regions, stream networks in the City of Worcester, MA have changed significantly as a result of development and urban growth. Increases in the percentage of impervious area due to development have generally resulted in increased peak flowrates with reduced time lags. High sediment and contaminant loadings in the urban drainage areas (associated with stormwater runoff, combined sewer overflows, and direct wastewater connections, etc.) adversely impact water quality of the local ponds and streams. Unfortunately, due to the variability of the flow and sediment loads, it is often difficult to quantify the nature of these loads and their impacts. Salisbury Pond, a small impounded surface water body located in Worcester, MA and shown in Figure 1, serves as an ideal example of a pond that has experienced these problems. The primary flow entering Salisbury Pond enters from Mill Brook, a culverted urban stream . The watershed for this pond is shown in Figure 2. Due to a long history of sediment and contaminant loading, the pond has experienced a reduction in volume and deterioration in water quality.

Objectives and methods

The objective of this work was to illustrate how watershed characteristics, field monitoring, sediment characterization, and hydrologic modeling can be combined to quantify loads entering a urbanized pond. A combination of watershed characterization, hydrologic model development, field measurements, and bottom sediment characterization was used to quantify flows and volumes and sediment loads entering the pond. Field measurements were obtained to characterize dry weather conditions and complete distributions of specific stormwater events. Laboratory protocols and analyses (e.g. total and volatile suspended solids, benthic sediment analyses, etc.) which were completed with assistance from D. Pellegrino, our laboratory technician, followed standard approaches (e.g. Standard Methods by Clescerl et al., 2002). Some of the metals results (characterized using an atomic absorption (A/A) spectrophotometer), are not shown here.

The HydroCAD software package, which includes algorithms for applying Natural Resources Conservation Service (NRCS) methods, was used to simulate runoff for specific storm events. Watershed characteristics provided basin parameters (e.g. runoff coefficients and times of concentration), and the basin was discretized to represent the effects of ponds the the nature of the drainage system as closely as possible.

Next steps

The next steps include refining and determining the applicability of these approaches, addressing organics, enhancing the modeling approaches to include sediments, metals and other contaminants, and using them to development BMP solutions. These issues are being addressed for other watersheds. These projects provide ideal mechanisms for introducing field and modeling experiences into the undergraduate and graduate curricula.

Text Box: Figure 2 – Land use for the Salisbury Pond Watershed

Text Box: Figure 1 – Aerial view of Salisbury Pond

Figure 3 - Schematic of modeling approach

Figure 4 - Estimated Bathymetry

Some results & implications

Examples of some of the data collected are shown in Figure 4. The data were used to refine the hydrologic model and understand loads and sediment transport for various conditions. By effectively integrating watershed characteristics (in terms of a GIS), field measurements (for dry and wet weather conditions), with pond bottom sediment characteristics and hydrologic model results, flowrates, sediment loads and contaminant loads can be estimated. These estimates can then provide a stronger basis for developing and evaluating BMP solutions.