Thomas J. Grizzard

Factors affecting the performance of stormwater treatment wetlands.

Data from 35 studies on 49 wetland systems used to treat stormwater runoff or runoff-impacted surface waters were examined and compared in order to identify any obvious trends that may aid future stormwater treatment wetland design efforts. Despite the intermittent nature of hydrologic and pollutant inputs from stormwater runoff, our analysis demonstrates that steady-state first-order plug-flow models commonly used to analyze wastewater treatment wetlands can be adapted for use with stormwater wetlands. Long-term pollutant removals are analyzed as functions of long-term mean hydraulic loading rate and nominal detention time. First-order removal rate constants for total phosphorus, ammonia, and nitrate generated in this fashion are demonstrated to be similar to values reported in the literature for wastewater treatment wetlands. Constituent removals are also demonstrated via regression analyses to be functions of the ratio of wetland area to watershed area. Resulting equations between these variables can be used as preliminary design tools in the absence of more site-specific details, with the understanding that they should be employed cautiously.

Distribution of polyaromatic hydrocarbons in the water column and sediments of a drinking water reservoir with respect to boating activity

Abstract The potential impact of motorboat activity on a major drinking water reservoir was evaluated. Polyaromatic hydrocarbons (PAHs) were measured in water and sediment samples at both marina and nonmarina sites during peak boating activity in June 1990 and during low boating activity in October 1990. Total PAH concentration contour plots and statistical analyses were used to determine the impacts of boating activity. Total PAH concentrations of ⩽4 μg/l were present in the water during peak boating activity, but aqueous PAHs were not detected during low boating activity. PAHs were detected in the sediments during both sampling periods; sediment concentrations for total PAHs were generally

Assessing performance of manufactured treatment devices for the removal of phosphorus from urban stormwater.

Nutrients such as nitrogen and phosphorus in urban runoff can be controlled through a variety of nonstructural and structural controls commonly known as best management practices (BMPs). Manufactured treatment devices (MTDs) are structural BMPs that may be used in portions of a site, often when space is limited. MTDs use a variety of technologies to achieve potentially greater treatment efficiency while reducing spatial requirements. However, verifying the performance of MTDs is difficult because of the variability of runoff water quality, the variability in treatment technologies, and the lack of standardized protocols for field testing. Performance testing of MTDs has focused almost exclusively upon removal of sediment; however MTDs are now being applied to the task of removing other constituents of concern, including nutrients such as phosphorus. This paper reviews current methods of assessing treatment performance of MTDs and introduces the Virginia Technology Assessment Protocol (VTAP), a program developed to evaluate the removal of phosphorus by MTDs. The competing goals of various stakeholders were considered when developing the VTAP. A conceptual framework of the tradeoffs considered is presented; these tradeoffs require compromise among the competing interests in order that innovation proceeds and benefits accrue. The key strengths of VTAP are also presented and compared with other existing programs.

Efficient design of stormwater holding basins used for water quality protection

Abstract By quantifying and taking advantage of the first flush phenomenon it is possible to develop a rational method for the design of stormwater basins based on the retention of a predetermined fraction of the pollutant load leaving a catchment. Such a procedure is applicable to the design of basins for small ( The data required for this design procedure is usually not available for most sites. Such data is both difficult and expensive to gather. However, as more data is obtained, it may be possible to develop generalizations among watersheds with regard to loading curves which are based on land use similar to land use groupings developed for the runoff coefficient “C”. Further, it may be possible to develop generalized timing curves for other pollutants. These curves are helpful in describing the runoff process and their development would provide additional capability in predicting the performance of stormwater basins. It is anticipated that the use of this method of design could result in substantially smaller basins being required for water quality protection in urban areas than would be the case if they were designed based strictly on the volume of runoff expected.

THE IMPACT OF ATMOSPHERIC CONTAMINANTS ON STORM WATER QUALITY IN AN URBAN AREA

Summary Composite precipitation samples were collected by recording raingages at nine stations in the Virginia portion of the greater Washington, D.C. metropolitan area. The samples were analyzed for COD, TOC, all nitrogen and phosphorus forms, and seven heavy metals; iron, zinc, lead, copper, cadmium, chromium and manganese. The results showed that wash out of the atmospheric contaminants occurred during the early stages of precipitation events, and that these contaminants are present in sufficient quantity to have a significant impact on surface water quality. Samples tended to contain similar amounts of pollutants after the same rainfall event regardless of land use location and distance from the center city up to 33 miles. Ranking techniques indicated, however, that precipitation in agricultural and land disturbance areas tends to contain more nutrients. Principal factors affecting the total quantity of pollutants in rainfall are atmospheric conditions (stagnant or turbulent) and antecedent conditions (time since previous precipitation event).

ALKALINITY AND TURBIDITY MEASUREMENTS AS FEEDBACK CONTROLS TO IMPROVE PERFORMANCE OF A BIOLOGICAL NITROGEN AND PHOSPHORUS REMOVAL SYSTEM

ABSTRACT Alkalinity and turbidity measurements on the aeration tank or secondary clarifier effluent provided suitable alternatives to dissolved oxygen (D.O.) measurements for controlling the oxygen supplied to a biological nitrogen and phosphorus removal system. An operating range was set up for effluent alkalinities and the oxygen supply was increased (or reduced) when the alkalinity rose above (or dropped below) this range. The effluent alkalinity was related to the extent of nitrification and denitrification. The effluent phosphorus levels increased along with turbidity when the amount of oxygen available was not sufficient for substrate stabilization and nitrification.

THE STIMULATION OF AUTOTROPHIC PRODUCTION BY URBAN STORMWATER-BORNE NUTRIENTS

SUMMARY Composite samples of stormwater runoff were collected from thirteen urban catchments representing six homogeneous land uses in the Virginia suburbs of Washington, D. C., USA. Phosphorus and nitrogen concentrations and availability in the stormwater samples were found to be sufficient to support moderate to high levels of production of the test alga, S. capricornutum . Lead and zinc concentrations in stormwater from certain urban land uses were found to be inhibitory to algal production. The annual average biomass to be supported by urban stormwater was found to bear a generally positive relationship to density of development.

THE SIGNIFICANCE OF PLANT NUTRIENT YIELDS IN RUNOFF FROM A MIXED LAND USE WATERSHED

Plant nutrient production was monitored during dry weather and storm flow conditions in seven sub-basins of the Occoquan Watershed. The areas monitored included land uses devoted to silviculture, agriculture, and suburban-urban development. Point source discharges were found to account for less than fifteen percent of total nutrient load during the period of minimum storm runoff. Drainage from urban land was found to export much more nitrogen and phosphorus than an equivalent amount of rural land. Log-linear relationships were observed to exist between plant nutrient loading(kg/ha) and runoff quantity (cm over the basin area) Log-linear regression models showed the effect of an impoundment at an intermediate point in the basin in reducing pollutant transport.

The Importance of Hydrologic Factors on the Relative Eutrophic Impacts of Point and Non-Point Pollution in a Reservoir

The Occoquan Reservoir, located downstream of a rapidly urbanizing Northern Virginia (USA) area, is a highly eutrophic water supply, which exhibits all the symptoms of excessive enrichment. Data collected during an intensive monitoring program since 1972 have vividly demonstrated the relative impacts of stormwater runoff and point-source sewage dis-charges on water quality.

Evaluating the performance of a retrofitted stormwater wet pond for treatment of urban runoff

This paper describes the performance of a retrofitted stormwater retention pond (Ashby Pond) in Northern Virginia, USA. Retrofitting is a common practice which involves modifying existing structures and/or urban landscapes to improve water quality treatment, often compromising standards to meet budgetary and site constraints. Ashby Pond is located in a highly developed headwater watershed of the Potomac River and the Chesapeake Bay. A total maximum daily load (TMDL) was imposed on the Bay watershed by the US Environmental Protection Agency in 2010 due to excessive sediment and nutrient loadings leading to eutrophication of the estuary. As a result of the TMDL, reducing nutrient and sediment discharged loads has become the key objective of many stormwater programs in the Bay watershed. The Ashby Pond retrofit project included dredging of accumulated sediment to increase storage, construction of an outlet structure to control flows, and repairs to the dam. Due to space limitations, pond volume was less than ideal. Despite this shortcoming, Ashby Pond provided statistically significant reductions of phosphorus, nitrogen, and suspended sediments. Compared to the treatment credited to retention ponds built to current state standards, the retrofitted pond provided less phosphorus but more nitrogen reduction. Retrofitting the existing stock of ponds in a watershed to at least partially meet current design standards could be a straightforward way for communities to attain downstream water quality goals, as these improvements represent reductions in baseline loads, whereas new ponds in new urban developments simply limit future load increases or maintain the status quo.