Robert Kröger

Agricultural Drainage Ditches Mitigate Phosphorus Loads as a Function of Hydrological Variability

Phosphorus (P) loading from nonpoint sources, such as agricultural landscapes, contributes to downstream aquatic ecosystem degradation. Specifically, within the Mississippi watershed, enriched runoff contributions have far-reaching consequences for coastal water eutrophication and Gulf of Mexico hypoxia. Through storm events, the P mitigation capacity of agricultural drainage ditches under no-till cotton was determined for natural and variable rainfall conditions in north Mississippi. Over 2 yr, two experimental ditches were sampled monthly for total inorganic P concentrations in baseflow and on an event-driven basis for stormflows. Phosphorus concentrations, Mannings equations with a range of roughness coefficients for changes in vegetative densities within the ditches, and discharge volumes from Natural Resources Conservation Service dimensionless hydrographs combined to determine ranges in maximum and outflow storm P loads from the farms. Baseflow regressions and percentage reductions with P concentrations illustrated that the ditches alternated between being a sink and source for dissolved inorganic P and particulate P concentrations throughout the year. Storm event loads resulted in 5.5% of the annual applied fertilizer to be transported into the drainage ditches. The ditches annually reduced 43.92 +/- 3.12% of the maximum inorganic effluent P load before receiving waters. Agricultural drainage ditches exhibited a fair potential for P mitigation and thus warrant future work on controlled drainage to improve mitigation capacity.

Ability of Four Emergent Macrophytes to Remediate Permethrin in Mesocosm Experiments

Increased focus is being placed on the ability of native vegetation to mitigate potential harmful effects of agricultural runoff, especially pyrethroid insecticides. Replicate 379xa0L Rubbermaid tubs (1.25xa0m [l]xa0×xa00.6xa0m [w]xa0×xa00.8xa0m [h]) were planted with individual species of cutgrass (Leersia oryzoides), cattails (Typha latifolia), bur-reed (Sparganium americanum), and powdery alligator-flag (Thalia dealbata), all common wetland macrophytes found in the Mississippi Delta, USA, agricultural region. Permethrin-enriched water (target concentration, 5xa0μg L−1) was pumped in at a 4-h hydraulic retention time at one end of the tub and discharged at the far end. Water samples were collected from discharge at 1-h intervals for 12xa0h and analyzed for permethrin concentrations. Permethrin removal rates were compared for the four different plant treatments and nonvegetated sediment-water controls. Results indicated that no particular single plant species was more effective at removing permethrin in water relative to unplanted controls. Overall mass reductions (from inflow to outflow) for cis-permethrin ranged from 67%xa0±xa06% in T. latifolia to 71%xa0±xa02% in L. oryzoides. The trans-permethrin overall mass reductions ranged from 76%xa0±xa04% in S. americanum to 82%xa0±xa02% in the unplanted control. Sediment and plant samples collected at the study conclusion indicated that 77%–95% of measured permethrin mass was associated with sediment for mesocosms planted with L. oryzoides, T. latifolia, and T. dealbata. Conversely, mesocosms planted with S. americanum had 83% of measured mass associated with the plant material. Specific plant-pesticide retention studies can lead to improved planning for best management practices and remediation techniques such as constructed wetlands and vegetated agricultural drainage ditches.

Inundation influences on bioavailability of phosphorus in managed wetland sediments in agricultural landscapes.

Agricultural runoff carries high nutrient loads to receiving waters, contributing to eutrophication. Managed wetlands can be used in integrated management efforts to intercept nutrients before they enter downstream aquatic systems, but detailed information regarding sorption and desorption of P by wetland sediments during typical inundation cycles is lacking. This study seeks to quantify and elucidate how inundation of wetland sediments affects bioavailability of P and contributions of P to downstream systems. A managed wetland cell in Tunica County, Mississippi was subjected to a simulated agricultural runoff event and was monitored for bioavailable phosphorus (water-extractable P [P], Fe-P, and Al-P) of wetland sediments and water level during the runoff event and for 130 d afterward. Inundation varied longitudinally within the wetland, with data supporting significant temporal relationships between inundation and P desorption. Concentrations of P were significantly higher at the site that exhibited variable hydroperiods (100 m) as compared with sites under consistent inundation. This suggests that sites that are inundated for longer periods of time desorb less P immediately to the environment than sites that have periodic or ephemeral inundation. Concentrations of iron oxalate and NaOH-P were significantly higher at the least inundated site as compared with all other sites (F = 5.43; = 0.001) irrespective of time. These results support the hypothesis that increased hydraulic residence time decreases the bioavailability of P in wetland sediments receiving agricultural runoff. This finding suggests that the restoration of wetlands in the mid-southern United States may be hydrologically managed to improve P retention.

Tiered on-the-ground implementation projects for Gulf of Mexico water quality improvements

Both the Gulf Hypoxia Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico and Improving Water Quality in the Mississippi River Basin (USEPA 2008) and the GOMA Governors Action Plan II for Healthy and Resilient Coasts (GOMA 2009) call for the development and implementation of nutrient reduction strategies to reduce excess nutrient loads to the Gulf of Mexico. However, it is not just the Gulf of Mexico waters that are impacted by excess nutrients. The US Environmental Protection Agency Wadeable Streams Assessment (USEPA 2006) and the National Lakes Assessment (USEPA 2010) indicated almost one-third of the nations stream miles and 20% of the lakes contain high total phosphorus (P) and total nitrogen (N) concentrations. Over 6,000 waterbodies in the United States are impaired by nutrients (USEPA 2011). The US Environmental Protection Agency Science Advisory Board has called for a 45% reduction of both N and P loads from the Mississippi River Basin to achieve the goal of a 5,000 km2 (1,930 mi2) hypoxic zone in the Gulf of Mexico (USEPA 2007). The approach taken by several states, including Mississippi, involves reduction of excess nutrients to attain the designated uses of state waterbodies for cumulative benefits…

Seasonal and Interspecific Nutrient Mitigation Comparisons of Three Emergent Aquatic Macrophytes

ABSTRACT The purpose of this study was to measure both summer and winter nutrient mitigation efficiencies of three aquatic plants found in agricultural drainage ditches in the lower Mississippi River Basin. Mesocosms (1.25 × 0.6 × 0.8 m) were filled with sediment and planted with monocultures of one of three obligate wetland plant species, Typha latifolia, Thalia dealbata, and Sagittaria latifolia, or left nonvegetated to serve as controls. Mesocosms were amended with nitrate, ammonium, and phosphate over a 4-h hydraulic retention time, followed by an 8-h flushing with nonamended water to assess residual nutrient leaching in both summer and winter exposures. Significant interactions between vegetation type and season were noted for both nitrate and total inorganic phosphorus concentrations and loads. Future research will focus on altering hydraulic retention time for improved efficiency, as well as the specific contribution of microbial activity to nutrient mitigation.

Utilization of common ditch vegetation in the reduction of fipronil and its sulfone metabolite

BACKGROUNDnFipronil, a phenylpyrazole insecticide, and its oxidative sulfone metabolite are two potential pollutants from treated rice and cotton production. A consequence of these pollutants occurring in surface runoff is degradation of downstream aquatic ecosystems. Utilization of primary intercept drainage ditches as management practices to reduce fipronil concentrations and loads has not been examined. This study used ditch mesocosms planted with monospecific stands of common emergent wetland vegetation to determine if certain plant species were more proficient in fipronil mitigation.nnnRESULTSnThree replicates of four plant species were compared against a non-vegetated control to determine differences in water column outflow concentrations (microg L(-1)) and loads (microg). There were no significant differences between vegetated and control treatments in outflow concentrations (F = 0.35, P = 0.836) and loads (F = 0.35, P = 0.836). The range of fipronil reduction was 28-45% for both concentration and load. Unlike fipronil, fipronil sulfone concentrations and load increased by 96-328%.nnnCONCLUSIONnThe increase in fipronil sulfone was hypothesized as a direct consequence of oxidation of fipronil within each mesocosm. The type of ditch vegetation had no effect on fipronil reduction. Future research needs to examine initial concentrations and hydraulic retention times to examine potential changes in reduction capacities.

Current- and past-use pesticide prevalence in drainage ditches in the Lower Mississippi Alluvial Valley

BACKGROUNDnPesticide application is common in agriculture and often results in applied pesticides entering adjacent aquatic systems. This study seasonally analyzed a suite of 17 current- and past-use pesticides in both drainage waters and sediments to evaluate the prevalence of pesticides in drainage ditches across the Lower Mississippi Alluvial Valley (LMAV).nnnRESULTSnThere were significantly higher concentrations (P<0.05) of current-use than past-use pesticides; however, there were consistently high numbers of detections of past-use pesticides in sediments. Sediment pesticide concentrations were an order of magnitude higher (150-1035 µg kg(-1)) than water samples (6-20.9 µg L(-1)). Overall, 87% of all samples analyzed for current- and past-use pesticides were non-detects. p,p-DDT was detected in 47.5% of all drainage waters and sediments sampled. There were significant correlations (0.372≥r2≤0.935) between detected current-use water and sediment concentrations, but no significant correlations between past-use water and sediment concentrations.nnnCONCLUSIONnOverall, there was a high percentage (87%) of sediment and water samples that did not contain detectable concentrations above the lower limit of analytical detection for each respective pesticide. This lack of pesticide prevalence highlights the improved conditions in aquatic systems adjacent to agriculture and a potential decrease in toxicity associated with pesticides in agricultural landscapes in the LMAV.

Efficiency of Experimental Rice (Oryza sativa L.) Fields in Mitigating Diazinon Runoff Toxicity to Hyalella azteca

This study assessed the viability of using planted, mature rice fields in mitigating diazinon (an organophosphate insecticide) runoff toxicity using aqueous 48xa0h Hyalella azteca whole effluent toxicity bioassays. Rice fields decreased diazinon concentrations 80.1%–99.9% compared with 10.8% in the unvegetated field control. H. azteca survival responses coincided with observed diazinon concentrations. Estimated LC50 effects dilutions (%) ranged from 1.15 to 1.47 for inflow samples and 1.66 (unvegetated), 6.44 (rice field A), and >100 (rice field B) outflow samples. Decreases in inflow versus outflow aqueous toxicity were 77.1%–100% in rice fields compared with 18.7% in the unvegetated field.

Tiered collaborative strategies for reducing hypoxia and restoring the Gulf of Mexico

The Gulf of Mexico is more than just a water body south of the United States. It is an international marine ecosystem, the ninth largest water body in the world (USEPA 2007), and it receives drainage from the Mississippi River Basin (MRB), the third largest drainage basin in the world. The combined gross domestic product of the five states bordering the Gulf of Mexico (Florida, Alabama, Mississippi, Louisiana, and Texas) makes this region the seventh largest global economy (IMF 2007). Major regional industries include commercial and recreational fishing, shrimp and oyster harvesting, tourism and recreation, and oil and gas production. Many of these industries are affected by the water quality of the Gulf of Mexico, as illustrated by the economic and environmental impacts of the 2010 Deepwater Horizon oil spill. Clearly, the health of the Gulf of Mexico contributes not only to the local and regional economy, but also to the national and global economy. Another phenomena annually affecting Gulf of Mexico water quality is the development of a “dead zone,” or area of hypoxia where dissolved oxygen concentrations decrease to levels that will not support aquatic life (Rabalais et al. 2002). This hypoxic zone is caused by nutrients transported…