Seven sets of ground water samples from 103 observation wells were analyzed for total dissolved phosphorus (TDP) in four areas and five materials including loess and loess derived alluvium in the Deep Loess Hills of western Iowa, outwash and fractured till adjacent to Clear Lake in north central Iowa, fractured till in central Iowa, and a sand and gravel aquifer in northwest Iowa. Land use in ground water recharge zones in all four areas is dominated by crop or animal production or both. Concentrations of TDP exceeding the minimum laboratory detection limit of 20 µg/l as P were found in all areas and in all materials sampled. Samples from the outwash deposits associated with Clear Lake contained significantly larger concentrations than all other areas and materials with a median of 160 µg/l. Water from fractured till in three areas produced the smallest range of concentrations with a median of 40 µg/l. The mean value of TDP in all sample sets exceeded 50 µg/l, an important ecological threshold that causes increased productivity in lakes and perennial streams and one being considered as a surface water nutrient standard by regulatory agencies. These results clearly show that ground water in essentially all near-surface aquifers and aquitards discharging to Iowa's streams and lakes is capable of sustaining P concentrations of 50 to 100 µg/l in streams, lakes, and reservoirs. Consequently, even if point discharges and sediment sources of P are substantially reduced, ground-water discharge to surface water may exceed critical thresholds under most conditions.
Herbicide transport in subsurface drainage can result in unacceptable levels of contamination in surface waters. This study assessed the extent of atrazine [6-chloro-N-ethyI-N'-(1-methylethyi)-l,3-,5-triazine-2,4-diamine] and metribuzin [4-amino-6-(1,1-dimethyethyl)-3-(methylthio)-l,2,4-triazin-5(4H)-one] transport to subsurface drainage and shallow groundwater. A corn (Zea mays L.) and soybean [Giycine max (L.) Merr.] rotation was used with corn receiving banded atrazine applications of 459 g ha-1 in 1992 and 561 g ha-1 in 1994. Soybean were treated with metribuzin at 420 g ha-~ in 1993 and 1995. Monthly flow-weighted average concentrations of atrazine in drainage water did not exceed 3 p~g L-1 and annual losses ranged from 0.02 to 2.16 g ha-1 during the 4-yr study. Less than 3% of the groundwater samples contained atrazine concentrations exceeding the 3 I~g L-~ maximum contaminant level for drinking water (MCL). Atrazine was detected more frequently in groundwater beneath the lowest parts of the field, despite greater than average sorption to soils in that area. Metribuzin was also found in groundwater,, but only half as frequently as atrazine. The patterns observed in subsurface drainage and groundwater reflected the persistence of atrazine and metribuzin in soil. Atrazine was detected in >90% of surface soil samples up to 23 mo after application, whereas metribuzin was rarely detected during the second year following application. Atrazine was found far more commonly than metribuzin in soil below 30 cm depth.
This study evaluated the influence of temporal variation on the occurrence, fate, and transport of tylosin (TYL) and sulfamethazine (SMZ); antibiotics commonly used in swine production. Atrazine (ATZ) was used as a reference analyte to indicate the agricultural origin of the antibiotics. We also assessed the impact of season and hydrology on antibiotic concentrations. A reconnaissance study of the South Fork watershed of the Iowa River (SFIR), was conducted from 2013 to 2015. Tile drain effluent and surface water were monitored using polar organic integrative sampler (POCIS) technology. Approximately 169 animal feeding operations (AFOs) exist in SFIR, with 153 of them being swine facilities. All analytes were detected, and detection frequencies ranged from 69 to 100% showing the persistence in the watershed. Antibiotics were detected at a higher frequency using POCIS compared to grab samples. We observed statistically significant seasonal trends for SMZ and ATZ concentrations during growing and harvest seasons. Time weighted average (TWA) concentrations quantified from the POCIS were 1.87ngL (SMZ), 0.30ngL (TYL), and 754.2ngL (ATZ) in the watershed. SMZ and TYL concentrations were lower than the minimum inhibitory concentrations (MIC) for E. coli. All analytes were detected in tile drain effluent, confirming tile drainage as a pathway for antibiotic transport. Our results identify the episodic occurrence of antibiotics, and highlights the importance identifying seasonal fate and occurrence of these analytes.
The livestock industry in the United States relies on antibiotics for disease prevention and treatment. As a result, antibiotic-laden manure is frequently applied to farmland to recycle nutrients. In the soil environment, antibiotics may accumulate and create selective pressure for antibiotic resistance in bacteria or travel to nearby water resources. This in vitro incubation study evaluated whether prairie buffer strips on farmland enhance degradation of three antibiotics-tetracycline, sulfamethazine, and tylosin-compared with degradation in soil from row crops adjacent to the strips.Soil from prairie strips of varying establishment ages and adjacent row crops were evaluated from three central Iowa sampling locations. Antibiotics mixed with swine manure slurry were added to soils at a starting concentration (10 μg kg -1 ) that reflects common veterinary antibiotic concentrations in soil and runoff after manure application. Antibiotic concentrations were quantified at six time points throughout a 72-d incubation period and fit to a first-order model to calculate decay rate constants and half-lives. The mean half-life for tetracycline was 0.54 d longer in prairie strip soil than row crop soil, whereas sulfamethazine and tylosin demonstrated no significant difference in persistence in strip or crop soil. Time since the establishment of the prairie strip did not affect antibiotic persistence. Concentrations of each antibiotic decreased to near-background levels throughout the incubation period. This study suggests that prairie strips do not consistently enhance antibiotic degradation in farm fields, but that antibiotics are unlikely to persist throughout the growing season in soil under strip or crop management.
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