Assessing sediment recontamination from metals in stormwater.

Recontamination of sediments by stormwater is a major concern when evaluating the potential effectiveness of sediment remediation. Stormwater and sediment sampling were conducted in a mixed-use watershed at Paleta Creek in San Diego, CA to evaluate methods for assessing sediment recontamination by metals. Size-segregated stormwater contaminant loads with simultaneous receiving water and sediment measurements were used to identify dominant sources and contaminants with respect to their impact on sediment recontamination. Most of the stormwater contaminant loads of Cd, Cu, Pb, and Zn were associated with residential and highway sources from the upstream portions of the watershed and As, Ni and Hg were more significantly influenced by the downstream area of the watershed. Cd was strongly associated with large particles (>63 µm) and observed to settle in near shore areas with some attenuation due to mixing and dilution. Cu, in contrast, was associated more with the filtered fraction (<0.45 µm) and clay fraction (0.45-5 µm), resulting in less near shore sediment recontamination. Depositing sediment and other metals, particularly Cu and Hg, exhibited greater accumulation in settling traps than could be attributed to stormwater loads indicating the importance of other sources or resuspension of bay sediments on surficial sediment concentrations. Pb, Zn, Ni, and As showed influences of both stormwater and other sources. The study showed that measurement of size-segregated stormwater contaminant mass and concentrations combined with simultaneous measurements of deposition in sediment traps could differentiate between recontamination by stormwater and that of other sources.

Statistical analyses of flow rates of stormwater treatment bioretention media.

The design and performance of stormwater controls is affected by the treatment flow rates of bioretention media. This article presents the results of a large number of laboratory column tests conducted to examine the treatment flow rates for various mixtures of stormwater bioretention media. Statistical analyses were conducted to identify the treatment media having targeted treatment flow rates. It was found that the bioretention media treatment flow rates were most affected by the median particle size (D50 ) and uniformity coefficient (Cu ) of the media, and the amount of organic matter. Statistical models were developed to evaluate and compare the treatment flow rates for various bioretention media mixtures. The findings of previous research (Sileshi, 2013) using two level, four factors (24 , with varying texture [T], uniformity [U], organic content [OC], and compaction [C]) full-factorial experiment study indicated that T and U of the media mixture have the greatest effect on the measured final infiltration rates of the media, followed by interactions of T and U; C; interactions of T and OC of the material; and interactions of U and OC of the material. As expected, media containing primarily larger particles (higher sand percentage) and, that is, uniformly graded (small uniformity coefficients) had the largest treatment flow rates. Compaction had minor effects if the organic matter content was low, but had significant effects on the flow rates for high organic matter content. PRACTITIONER POINTS: Bioretention media treatment flow rates were most affected by the median particle size and uniformity coefficient of the media, and the amount of organic matter. Statistical models were developed to evaluate and compare the treatment flow rates for various bioretention media mixtures. The media containing primarily larger particles and, that is, uniformly graded (small uniformity coefficients) had the largest treatment flow rate.

Targeting treatment technologies to address specific stormwater pollutants and numeric discharge limits.

Stormwater treatment is entering a new phase with stormwater management systems being required to meet specific numeric objectives, as opposed to the historic approach of meeting guidance-document-provided percent removal rates. Meeting numeric discharge requirements will require designers to better understand and apply the physical, chemical, and biological processes underpinning these treatment technologies. This critical review paper focuses on the potential unit treatment operations available for stormwater treatment and outlines how to identify the most applicable treatment options based on the needed pollutant removal goals.

Peristaltic pump autosamplers for solids measurement in stormwater runoff.

Regulatory agencies approve automatic samplers containing peristaltic pumps as a sample collection method for stormwater characterization and for treatment-device evaluation. Autosampler performance, as discussed in the limited available literature, can vary across the entire particle size range typically found in stormwater from different source areas and outfalls-reasonably consistent performance for particle sizes < 250 microm, but much less consistency for particles > 250 microm. Therefore, a series of experiments was undertaken to quantify the upper range of consistent particle capture that may occur with sampling stormwater suspended sediment and particulate-bound pollutants. These experiments, based on triplicate sampling at each experimental condition, found that peristaltic pump autosamplers commonly used in stormwater monitoring could not repeatedly and effectively capture particles > 250 microm from a simulated stormwater whose particles have a specific gravity of 2.65. It was expected that the effective size for autosamplers would be correspondingly larger for particles having smaller specific gravities. The height of the sampler had no influence on particle recovery up to a height of 2.5 m, with slightly decreasing recoveries of large particles occurring at greater heights, as a result of reduced sampler intake velocity. Therefore, to characterize the solids across the entire size range and specific gravities that may occur in stormwater runoff, autosamplers should be deployed in conjunction with bedload and floatables sampling.

Influencing factors and a proposed evaluation methodology for predicting groundwater contamination potential from stormwater infiltration activities.

To offset the detrimental effects of urbanization on groundwater recharge, stormwater managers are focusing on infiltrating much of the runoff from a site that was generated because of development. For this to be effective, tools are required to predict the potential for contamination resulting from this infiltration for many site conditions, because infiltration should be stressed in areas where the least potential for causing groundwater contamination exists. Factors that influence contamination potential include the pollutant concentration in the runoff directed to the infiltration device and the ability of the underlying soil to remove the pollutant. The groundwater contamination potential of some pollutants, even those with high concentrations and moderate-to-high mobilities, can be reduced with proper pretreatment before infiltration. This paper presents a methodology that can be used to evaluate infiltration as an management option and introduces two different levels of models that could be used to evaluate contamination potential.