High levels of accumulated phosphorus (P) in soils of the Delmarva Peninsula are a major source of dissolved P entering drainage ditches that empty into the Chesapeake Bay. Th e objective of this study was to design, construct, and monitor a within-ditch fi lter to remove dissolved P, thereby protecting receiving waters against P losses from upstream areas. In April 2007, 110 Mg of fl ue gas desulfurization (FGD) gypsum, a low-cost coal combustion product, was used as the reactive ingredient in a ditch fi lter. Th e ditch fi lter was monitored from 2007 to 2010, during which time 29 storm-induced fl ow events were characterized. For storm-induced fl ow, the event mean concentration effi ciency for total dissolved P (TDP) removal for water passing through the gypsum bed was 73 ± 27% confi dence interval (α = 0.05). Th e removal effi ciency for storm-induced fl ow by the summation of load method was 65 ± 27% confi dence interval (α = 0.05). Although chemically eff ective, the maximum observed hydraulic conductivity of FGD gypsum was 4 L s −1 , but it decreased over time to <1 L s −1 . When bypass fl ow and base fl ow were taken into consideration, the ditch fi lter removed approximately 22% of the TDP load over the 3.6-yr monitoring period. Due to maintenance and clean-out requirements, we conclude that ditch fi ltration using FGD gypsum is not practical at a farm scale. However, we propose an alternate design consisting of FGD gypsum-fi lled trenches parallel to the ditch to intercept and treat groundwater before it enters the ditch.

Rainfall is a driving force for the transport of environmental contaminants from agricultural soils to surficial water bodies via surface runoff.

Urea-N is linked to harmful algal blooms in lakes and estuaries, and urea-N-based fertilizers have been implicated as a source. However, the export of urea-N-based fertilizers appears unlikely, as high concentrations of urea-N are most commonly found in surface waters outside periods of fertilization. To evaluate possible autochthonous production of urea-N, we monitored urea-N released from drainage ditch sediments using mesocosms. Sediments from a cleaned (recently dredged) drainage ditch, uncleaned ditch, forested ditch, riparian wetland, and an autoclaved sand control were isolated in mesocosms and flooded for 72 h to quantify urea-N, NH 4 + -N, and NO 3 --N in the floodwater. Sediments were flooded with different N-amended-N) and incubated at three water temperatures (16, 21, and 27°C). Urea-N concentrations in mesocosms representing uncleaned and cleaned drainage ditches were significantly greater than nonagricultural sediments and controls. While flooding sediments with N-enriched solution had no clear effect on urea-N, warmer (27°C) temperatures resulted in significantly higher urea-N. Data collected from field ditches that were flooded by a summer rainstorm showed increases in urea-N that mirrored the mesocosm experiment. We postulate that concentrations of urea-N in ditches that greatly exceed environmental thresholds are mediated by biological production in sediments and release to stagnant surface water. Storm-driven urea-N export from ditches could elevate the risk of harmful algal blooms downstream in receiving waters despite the dilution effect. (Glibert et al., 2004). This phenomenon has drawn attention to sources of urea-N to surface water, and elevated urea-N concentrations issuing from agricultural watersheds have led some researchers to suggest a link to the land application of manures and urea-N synthetic fertilizers (Glibert et al., 2001(Glibert et al., , 2005(Glibert et al., , 2006Lomas et al., 2002;Thorén et al., 2003). Indeed, global urea-N fertilizer consumption ballooned 100-fold from the 1960s to the 2000s (Glibert et al., 2006), part of a trend of dramatically increasing fertilizer N use since the mid-twentieth century. Due in part to restrictions on inorganic fertilizers because of security concerns, urea-N comprises perhaps 60% of global N fertilizer use on an annual basis, and this proportion is expected to grow (Glibert et al., 2014). However, although small losses of agriculturally applied urea-N could be biologically important in receiving waters (Glibert et al., 2006), the export of untransformed urea-N fertilizer is likely minor. The rapid hydrolysis of urea-N in soils generally precludes substantial export from fertilizer applications (Fisher et al., 2016). Losses via leaching and overland flow are brief and typically small even under heavy rainfall conditions (Han et al., 2015;Kibet et al., 2016). In a synoptic watershed study on the Delmarva Peninsula, Tzilkowski (2013) found no evidence that storms in the weeks after poultry manure application led to increased ur...

Studies of harmful algal blooms and associated urea concentrations in the Chesapeake Bay and in coastal areas around the globe strongly suggest that elevated urea concentrations are associated with harmful algal blooms. The observed increased frequency and toxicity of these blooms in recent decades has been correlated with increased agricultural use of N inputs and increased use of urea as a preferred form of commercial N. This rainfall simulation study sought to assess the potential for different N fertilizers and manures to contribute to urea in runoff from a Coastal Plain soil on the Eastern Shore of Maryland. Under worst-case conditions, ?1% of urea-N applied as commercial fertilizer and surface-applied poultry litter was lost in runoff in a simulated rainfall event, roughly equivalent to a 1-yr return period rain storm in the study area, 12 h after application. Cumulative urea-N losses, including four subsequent weekly rainfall events, approached 1.7% from urea-N fertilizer containing a urease inhibitor. Urea-N loss from incorporated poultry litter was negligible, and losses from dairy manure were intermediate. These losses are likely confined to hydrological contributing areas that extend several meters from a drainage ditch or stream for storms with frequent recurrence intervals. Cumulative dissolved N losses in runoff (urea-N + ammonium-N + nitrate-N) as a proportion of total applied plant-available N were <5%, suggesting that most of the applied N was lost by other pathways or was immobilized in soil. Results also highlight the potential for simple management options, such as shallow incorporation or timing, to greatly reduce urea runoff losses.