Evaluation of Practices to Remove Nutrients and Sediment from Stormwater Runoff

Abstract

Urban development contributes to increased pollutant loading. This includes sediment from construction sites and nutrients from impervious-surface runoff. Urban development has increased construction rates and the percentage of impervious cover. To help manage runoff, stormwater control measures (SCMs) are implemented, reducing runoff volumes and improving water quality. Nutrients, such as nitrogen and phosphorus, are considered harmful pollutants in stormwater, yet are vital and limited resources for agriculture. To address concerns about high nutrient loading from stormwater and provide an alternative source, this study aimed to evaluate the performance of a standard infiltration trench stormwater control measure (SCM) and two additional modifying components: an iron-modified geotextile (FeGeotex) and iron-enhanced sand. These designs were tested independently within a lab-scale infiltration trench (8.0 ft [2.44m] x 2.5 ft [0.76 m] x 4.0 ft [1.22 m]) with synthetic stormwater under low-concentration (blank), stormwater-concentration (dosed), and high-concentration (stress) conditions. The concentrations of nitrogen and phosphorus in the outflow were measured and compared with those in the inflow to determine removal rates. The rates were compared using an ANOVA and a two-means t-test. Testing showed that changing the designs had no significant impact on nitrogen removal under blank, dosed, or stress conditions. The standard infiltration trench had average phosphorus removal rates of -4.3% (leached), 32.2%, and 40.2% for the blank, dosed, and stress conditions, respectively. The iron-enhanced sand showed the greatest phosphorus removal across all three conditions, with average removal rates of 38.9%, 76.9%, and 82.1% for the blank, dosed, and stress conditions, respectively. The FeGeotex had significantly greater removal than the standard under both blank and dosed conditions, with removals of 13.8% and 42.5%, but was not significantly different from the standard under stress conditions, with 43.9% removal. All three designs proved capable of recovering phosphorus for reuse. The geotextile from the standard design recovered 0.033 mg-P/g-geotextile. The FeGeotex recovered 0.059 mg-P/g-geotextile, the top geotextile from the iron-enhanced sand design recovered 0.053 mg-P/g-geotextile, the iron-enhanced sand recovered 0.075 mg-P/g-media, and the bottom geotextile from the iron-enhanced sand design had the greatest phosphorus recovery with 0.102 mg-P/g-geotextile. Both modification components proved capable of enhancing phosphorus removal and providing phosphorus for reuse, with the iron-enhanced sand performing best for both criteria. To remove sediment from runoff and prevent loss of sediment off-site, perimeter controls, such as sediment barriers, are typically implemented. To evaluate sediment barriers and compare their performance under different conditions, testing was conducted at the Auburn University – Stormwater Research Facility. Testing was conducted on two novel products: Product A and Product B, and three standard practices: silt fence, compost socks, and straw wattles. The silt fence and both products were subjected to triplicate back-to-back simulated storms for each of three installations, and Product A, the compost socks, and straw wattles were subjected to a single storm for each of three installations, as well as installed with additional impoundment area. For the triplicate testing, the silt fence had the greatest average sediment capture at 97%, followed by Product B at 88% and Product A at 74%. For single-storm testing, the compost sock had the highest sediment capture at 95%, followed by the straw wattle and Product A, both at 90%. A comparison of sediment capture for Product A under both testing regimes showed that the product performs statistically better with an increased impoundment area and with sufficient maintenance. A comparison of performance between the products and prior testing on various wattle designs and installation methods showed no significant difference in sediment capture. This testing showed that the products perform effectively and are similar to standard practices when installed correctly and maintained regularly. These findings indicate that enhanced nutrient removal and recovery for reuse is plausible using iron-based modifying components, especially iron-enhanced sand. Testing of sediment barrier products and standard practices showed that installation and practice selection are important for effective use of sediment barriers as perimeter controls on construction sites.