Using flue gas desulfurization gypsum to remove dissolved phosphorus from agricultural drainage waters.

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. The objective of this study was to design, construct, and monitor a within-ditch filter to remove dissolved P, thereby protecting receiving waters against P losses from upstream areas. In April 2007, 110 Mg of flue gas desulfurization (FGD) gypsum, a low-cost coal combustion product, was used as the reactive ingredient in a ditch filter. The ditch filter was monitored from 2007 to 2010, during which time 29 storm-induced flow events were characterized. For storm-induced flow, the event mean concentration efficiency for total dissolved P (TDP) removal for water passing through the gypsum bed was 73 ± 27% confidence interval (α = 0.05). The removal efficiency for storm-induced flow by the summation of load method was 65 ± 27% confidence interval (α = 0.05). Although chemically effective, 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 flow and base flow were taken into consideration, the ditch filter 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 filtration using FGD gypsum is not practical at a farm scale. However, we propose an alternate design consisting of FGD gypsum-filled trenches parallel to the ditch to intercept and treat groundwater before it enters the ditch.

Examining sources and pathways of phosphorus transfer in a ditch-drained field.

Understanding the processes that mobilize and transport dissolved phosphorus (P) during storms is critical to managing P in flat landscapes with open ditch drainage and legacy soil P. In this study, we used routine baseflow monitoring and intensive storm sampling at a ditch-drained site on Maryland's Lower Eastern Shore (July 2017-September 2018) to assess whether concentration-discharge (C-Q) relationships and chemical and isotopic hydrograph separation could provide insight into the processes that mobilize and transport dissolved P in ditch drainage. Using a segmented regression model, we determined that long-term C-Q relationships for dissolved P differed above and below a discharge threshold of 6.4 L s-1 . Intensive storm sampling revealed that small storms (n = 3) occurring at or below the discharge threshold generally exhibited complex hysteresis and dissolved P dilution patterns that were consistent with deeper (>122 cm) groundwater inputs with low dissolved P concentrations (0.04 mg L-1 ). In contrast, large storms occurring well above the discharge threshold (n = 4) induced rising water tables and preferential flow pathways that most likely tapped dissolved P-enriched shallow (<20 cm) soil waters (0.89 mg L-1 ), producing consistent clockwise hysteresis and dissolved P flushing patterns. Notably, chemical and isotope hydrograph separation during two of the largest storms revealed significant event water fractions (59-68%) that strongly suggested a role for the rapid delivery of dissolved P via preferential flow pathways. Findings highlight the need to mitigate vertical P stratification as a means for reducing dissolved P flushing from ditch-drained landscapes with legacy P.

Baseflow nitrate in relation to stream order and agricultural land use.

Management of agricultural nonpoint-source pollution continues to be a challenge because of spatial and temporal variability. Using stream order as an index, we explored the distribution of nitrate concentration and load along the stream network of a large agricultural watershed in Pennsylvania-the East Mahantango Creek Watershed and two of its sub-watersheds. To understand nitrate concentration variation in the stream water contributed from ground water, this study focused on baseflow. Impacts of agricultural land use area on baseflow nitrate in the stream network were investigated. Nitrate concentration showed a general decreasing trend with increasing stream order based on stream order averaged values; however, considerable spatial and temporal variability existed within each snapshot sampling. Nitrate loads increased with stream order in a power function because of the dominant effect of stream flow rate over the nitrate concentration. Within delineated sub-watersheds based on stream orders, positive linear functions were found between agricultural land use area percentage and the baseflow nitrate concentration and between agricultural drainage area and the nitrate load. The slope of the positive linear regression between the baseflow nitrate concentration and percent agricultural land area seems to be a valuable indicator of a watershed's water quality as influenced by agricultural practices, watershed size, and specific physiographic setting. Stream order seems to integrate, to a certain degree, the source and transport aspects of nonpoint-source pollution on a yearly averaged basis and thus might provide a quick estimate of the overall trend in baseflow nitrate concentration and load distribution along complex stream networks in agricultural watersheds.

Phosphorus loss from an agricultural watershed as a function of storm size.

Phosphorus (P) loss from agricultural watersheds is generally greater in storm rather than base flow. Although fundamental to P-based risk assessment tools, few studies have quantified the effect of storm size on P loss. Thus, the loss of P as a function of flow type (base and storm flow) and size was quantified for a mixed-land use watershed (FD-36; 39.5 ha) from 1997 to 2006. Storm size was ranked by return period (<1, 1-3, 3-5, 5-10, and >10 yr), where increasing return period represents storms with greater peak and total flow. From 1997 to 2006, storm flow accounted for 32% of watershed discharge yet contributed 65% of dissolved reactive P (DP) (107 g ha(-1) yr(-1)) and 80% of total P (TP) exported (515 g ha(-1) yr(-1)). Of 248 storm flows during this period, 93% had a return period of <1 yr, contributing most of the 10-yr flow (6507 m(3) ha(-1); 63%) and export of DP (574 g ha(-1); 54%) and TP (2423 g ha(-1); 47%). Two 10-yr storms contributed 23% of P exported between 1997 and 2006. A significant increase in storm flow DP concentration with storm size (0.09-0.16 mg L(-1)) suggests that P release from soil and/or area of the watershed producing runoff increase with storm size. Thus, implementation of P-based Best Management Practice needs to consider what level of risk management is acceptable.

Integrating contributing areas and indexing phosphorus loss from agricultural watersheds.

Most states in the USA have adopted P Indexing to guide P-based management of agricultural fields by identifying the relative risk of P loss at farm and watershed scales. To a large extent, this risk is based on hydrologic principles that frequently occurring storms can initiate surface runoff from fields. Once initiated, this hydrological pathway has a high potential to transport P to the stream. In regions where hydrologically active areas of watersheds vary in time and space, surface runoff generation by "saturation excess" has been linked to distance from stream, with larger events resulting in larger contributing distances. Thus, storm-return period and P loss from a 39.5-ha mixed-land-use watershed in Pennsylvania was evaluated to relate return-period thresholds and distances contributing P to streams. Of 248 storm flows between 1997 and 2006, 93% had a return period of 1 yr, contributing 47% of total P (TP) export, while the largest two storms (10-yr return period) accounted for 23% of TP export. Contributing distance thresholds for the watershed were determined (50-150 m) for a range of storm-return periods (1-10 yr) from hydrograph analysis. By modifying storm-return period thresholds in the P Index and thereby contributing distance, it is possible to account for greater risk of P loss during large storms. For instance, increasing return period threshold from 1 (current P indices) to 5 yr, which accounted for 67% of TP export, increased the P-management restricted area from 20 to 58% of the watershed. An increase in impacted area relative to a decreased risk of P loss creates a management-policy dilemma that cannot be ignored.

Imaging Hydrological Processes in Headwater Riparian Seeps with Time-Lapse Electrical Resistivity.

Delineating hydrologic and pedogenic factors influencing groundwater flow in riparian zones is central in understanding pathways of water and nutrient transport. In this study, we combined two-dimensional time-lapse electrical resistivity imaging (ERI) (depth of investigation approximately 2 m) with hydrometric monitoring to examine hydrological processes in the riparian area of FD-36, a small (0.4 km2 ) agricultural headwater basin in the Valley and Ridge region of east-central Pennsylvania. We selected two contrasting study sites, including a seep with groundwater discharge and an adjacent area lacking such seepage. Both sites were underlain by a fragipan at 0.6 m. We then monitored changes in electrical resistivity, shallow groundwater, and nitrate-N concentrations as a series of storms transitioned the landscape from dry to wet conditions. Time-lapse ERI revealed different resistivity patterns between seep and non-seep areas during the study period. Notably, the seep displayed strong resistivity reductions (∼60%) along a vertically aligned region of the soil profile, which coincided with strong upward hydraulic gradients recorded in a grid of nested piezometers (0.2- and 0.6-m depth). These patterns suggested a hydraulic connection between the seep and the nitrate-rich shallow groundwater system below the fragipan, which enabled groundwater and associated nitrate-N to discharge through the fragipan to the surface. In contrast, time-lapse ERI indicated no such connections in the non-seep area, with infiltrated rainwater presumably perched above the fragipan. Results highlight the value of pairing time-lapse ERI with hydrometric and water quality monitoring to illuminate possible groundwater and nutrient flow pathways to seeps in headwater riparian areas.