Flow-weighted Mean Concentrations Research Articles

Ultrafine (

Ultrafine particulate SRP may have an important role in the transfer of P from the Everglades Stormwater Treatment Areas (STAs), constructed wetlands designed to reduce phosphorus (P) in stormwater that is discharged to the Everglades Protection Area, to sensitive downstream water bodies due to increases in mobility associated with small particle size. P concentrations in discharges from the STAs are monitored using routine methods for particulate P (>0.45 μm) and dissolved P (<0.45 μm), an approach that does not consider ultrafine particulate P within the <0.45 μm fraction. Ultrafine particulate SRP was determined by ultrafiltration and was operationally defined as SRP associated with particles between <0.45 μm and <3 kDa. This study determined the magnitude and proportion of ultrafine particulate SRP within the <0.45 μm fraction of inflow, outflow, and in some cases, midflow waters from a selection of well-performing (discharge flow weighted mean concentration (FWMC) of P ≤ 20 μg L−1) and under-performing flow-ways (FWs) (FWMC P > 20 μg L−1) containing emergent aquatic vegetation (EAV), submerged aquatic vegetation (SAV) or a mix of EAV/SAV during the wet and dry season. During the wet season, inflow <0.45 μm SRP was predominately (60–100%) truly dissolved SRP (< 3 kDa) in all flow-ways. In the under-performing flow-ways, <0.45 μm SRP remained largely as truly dissolved, highly available SRP from inflow to outflow, whereas in the well-performing flow-ways, the percentage of <0.45 μm SRP present as ultrafine particulate SRP increased up to 100% as the distance from the inflow increased and the amount of truly dissolved SRP was very low. Two flow-ways having 100% particulate SRP at the outflow (STA-3/4 CFW and STA-5/6 FW1) contain an EAV cell followed by an EAV/SAV cell. However, in flow-ways containing more SAV (STA-2 FW3), ultrafine particulate SRP was a smaller proportion of outflow SRP (29%). During the dry season, ultrafine particulate SRP similarly increased from inflow to outflow at some well and under-performing sampling locations, but to a smaller proportion, and ultrafine particulate SRP never made up 100% of the <0.45 μm SRP fraction. These findings reveal that ultrafine particulate SRP is largely present within the <0.45 μm SRP fraction, at times making up >50% of TP, making it an important but poorly understood component of STA P budgets.

Improved Darcian streambed measurements to quantify flux and mass discharge of volatile organic compounds from a contaminated aquifer to an urban stream

Quantifying VOC transport from contaminated groundwater to streams is challenging and important for understanding off-site migration of VOCs, cross-media contamination (groundwater to surface water and eventually air), and potential impacts on downstream ecosystems and human populations. A streambed point sampling approach was used to quantify fluxes of water and 14 VOCs from groundwater to an urban stream in North Carolina, USA, during summer (June 2015) and winter (January 2016). The approach is unique in coupling measurements of vertical hydraulic conductivity, vertical hydraulic head gradient, and groundwater VOC concentration at each individual sampling point, reducing or eliminating some potential concerns with other Darcian methods for quantifying VOC inputs to streams.Most results were consistent with discharge of two main VOC plumes on opposite sides of the stream. Plume 1 from the west side was dominated by cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) at mean concentrations of 19 and 11 μg L−1, respectively. Plume 2 from the east side was dominated by benzene (mean concentration 56 μg L−1). Plume 2 was not previously known, and the improved sampling approach allowed VOC discharge from both plumes to be quantified simultaneously.For 13 of the 14 detected VOCs, the mean VOC flux from groundwater to the stream (fVOC) was higher in January 2016 than in June 2015, mainly because groundwater flux was higher in January. The only exception was cDCE, the most abundant VOC in Plume 1, which had mean fVOC values of 9.8 and 9.5 mg m−2 d−1 in June 2015 and January 2016, respectively. Benzene was the most abundant VOC in Plume 2 and had mean fVOC values of 11 and 37 mg m−2 d−1 in June 2015 and January 2016, respectively. High groundwater flux drove almost all the occurrences of high VOC flux. For a given VOC, the flow-weighted mean concentration (with each VOC concentration weighted by the upward groundwater flux at the VOC sampling point) was generally larger than the unweighted mean concentration. Thus, flow-weighting of concentrations gave a more accurate indication of the average VOC concentration in net groundwater discharge to the stream.An estimate of total VOC mass discharge from groundwater to the study reach of the stream, 3.6 kg of VOC per year, was based on the fVOC results and streambed area in the reach. The bulk of this discharge was due to benzene, cDCE, and VC, with individual mass discharges of 2.1, 0.83, and 0.40 kg yr−1, respectively. Estimates of maximum potential VOC degradation in the streambed suggest that the 3.6 kg yr−1 estimate of mass discharge was not sensitive to potential degradation of VOCs in the streambed sediments above the groundwater sampling depth.

The spatial organization of CAFOs and its relationship to water quality in the United States

• Applied Nearest Neighbor Index and Moran’s I to quantify the spatial pattern of AFOs. • Watersheds with higher AFO NNI were associated with elevated nutrient levels. • Clustering of smaller farms (less regulated) also corresponded to high nutrients. • Results help to inform management of AFOs and other pollutant sources. Large animal operations in the United States (US) have expanded in recent decades as a solution for meeting increasing food demand. These operations can release large amounts of organic matter and nutrients into the environment, potentially impacting water quality conditions of nearby waterbodies. Often referred to as animal feeding operations (AFOs) or CAFOs (concentrated AFOs) based on nomenclature in federal regulations, these operations are usually clustered in space, which may intensify the impacts observed. However, the influence of the known spatial aggregation of CAFOs on water quality conditions has not been evaluated. This study investigates the spatial organization of CAFOs and its relationship to water quality conditions. We hypothesized that watersheds with clustered CAFOs are likely to be associated with higher concentrations of Total Phosphorus (TP) and Total Nitrogen (TN) in the US. We gathered and digitized CAFO locations in 16 US states and used TP and TN concentration data to evaluate our hypothesis. We calculated the flow-weighted mean concentration of TN and TP from 2005 to 2014 and averaged those values per season. We used the nearest neighbor index (NNI) to determine the spatial pattern (i.e., clustering or dispersed) of CAFOs per watershed and its significance level. Additionally, for states with animal number information per farm, we performed a Local Moran’s I analysis to understand if the clustering of CAFOs with a low or high number of animals relates to the TP and TN concentrations of each nearest water quality station. Overall, we found that watersheds with significant clustering patterns were associated with higher TP and TN levels. Seasonally, this finding also held principally during spring and summer seasons, when manure tends to be land applied. Our Local Moran’s I analysis also revealed that the clustering of smaller and medium-sized farms, usually less regulated, is associated with elevated TP and TN concentrations. This reveals that the small, yet potentially unregulated farms could be highly impactful to the environment, which warrants further study. Given the lack of information on the influence of CAFO clustering (and other pollutant source clustering) on water quality and that most states do not regulate emitters based on this aspect, our outcomes advance the knowledge of how these entities may be driving changes to the environment. This study also brings insights into new water quality modeling approaches and supports future policy decisions.

Evaluation of Chemcatcher® passive samplers for pesticide monitoring using high-frequency catchment scale data

Passive samplers (PS) have been proposed as an enhanced water quality monitoring solution in rivers, but their performance against high-frequency data over the longer term has not been widely explored. This study compared the performance of Chemcatcher® passive sampling (PS) devices with high-frequency sampling (HFS: 7-hourly to daily) in two dynamic rivers over 16 months. The evaluation was based on the acid herbicides MCPA (2-methyl-4-chlorophenoxyacetic acid), mecoprop-P, fluroxypyr and triclopyr. The impact of river discharge parameters on Chemcatcher® device performance was also explored. Mixed effects modelling showed that time-weighted mean concentration (TWMC) and flow-weighted mean concentration (FWMC) values obtained by the HFS approach were both significantly higher (p < 0.001) than TWMC values determined from PS regardless of river or pesticide. Modelling also showed that TWMCPS values were more similar to TWMCHFS than FWMCHFS values. However, further testing revealed that MCPA TWMC values from HFS and PS were not significantly different (p > 0.05). There was little indication that river flow parameters altered PS performance—some minor effects were not significant or consistent. Despite this, the PS recovery of very low concentrations indicated that Chemcatcher® devices may be used to evaluate the presence/absence and magnitude of acid herbicides in hydrologically dynamic rivers in synoptic type surveys where space and time coverage is required. However, a period of calibration of the devices in each river would be necessary if they were intended to provide a quantitative review of pesticide concentration as compared with HFS approaches.

High frequency UV–Vis sensors estimate error in riverine dissolved organic carbon load estimates from grab sampling

High frequency ultraviolet – visible (UV–VIS) sensors offer a way of improving dissolved organic carbon (DOC) load estimates in rivers as they can be calibrated to DOC concentration. This is an improvement on periodic grab sampling, or the use of pumped sampling systems which store samples in-field before collection. We hypothesised that the move to high frequency measurements would increase the load estimate based on grab sampling due to systemic under-sampling of high flows. To test our hypotheses, we calibrated two sensors in contrasting catchments (Exe and Bow Brook, UK) against weekly grab sampled DOC measurements and then created an hourly time series of DOC for the two sites. Taking this measurement as a ‘true’ value of DOC load, we simulated 1,000 grab sampling campaigns at weekly, fortnightly and monthly frequency to understand the likely distribution of load and error estimates. We also performed an analysis of daily grab samples collected using a pumped storage sampling system with weekly collection. Our results show that: a) grab sampling systemically underestimates DOC loads and gives positively skewed distributions of results, b) this under-estimation and positive skew decreases with increasing sampling frequency, c) commonly used estimates of error in the load value are also systemically lowered by the oversampling of low, stable flows due to their dependence on the variance in the flow-weighted mean concentration, and d) that pumped storage systems may lead to under-estimation of DOC and over estimation of specific ultra-violet absorbance (SUVA), a proxy for aromaticity, due to biodegradation during storage.

The seasonality of nitrate and phosphorus leaching from manure and chemical fertilizer added to a chernozemic soil in Canada.

Identifying seasons sensitive to nutrient losses could help farmers and policymakers to formulate effective nutrient loss reduction strategies. This long-term study monitored water percolation as well as nitrate (NO3 -N) and total phosphorus (TP) leaching from liquid swine manure and chemical fertilizer applied to intact core lysimeters in a sandy loam soil in Manitoba, Canada. Water percolation, NO3 -N, and TP leaching were monitored from 2005 to 2016. Chemical fertilizer showed greater average annual mean water percolation (p=.01), annual flow-weighted mean concentration (FWMC) of NO3 -N (22mg L-1 ; p<.001), and annual NO3 -N leaching (36kg N ha-1 ; p=.002) compared with the manure treatment (FWMC NO3 -N, 15mg L-1 ; NO3 -N leaching load, 22kg N ha-1 ). Average annual mean TP loss did not differ between treatments (p=.86). Spring (April-June) was the most sensitive season, when>75% of annual percolation,>80% of annual NO3 -N, and>68% of annual TP leaching losses occurred from both manure and chemical fertilizer. Annual NO3 -N and TP leaching increased exponentially with cumulative winter and spring precipitation (control, r2 =.69; manure, r2 =.79; chemical fertilizer, r2 =.63) and decreased with winter and spring air temperatures. The largest spring NO3 -N and TP leaching losses were observed in 2013, which followed the dry year of 2012, indicating the potential for nutrient flushing. The findings emphasize the need for environmentally sound N and P management strategies in cold North American regions underlain by coarse-textured soils, particularly during the spring season.

Importance of Flood Samples for Estimating Sediment and Nutrient Loads in Mediterranean Rivers

Protecting the quality of coastal water bodies requires the assessment of contaminant discharge brought by rivers. Numerous methods have been proposed for calculating sediment and nutrient loads. The most widely used and generally recommended are the flow-weighted mean concentration method (FWMC) and the flow duration rating curve method (FDRC). In the Mediterranean basin, the hydrology is characterized by infrequent but very intense rainfall events. The flows taking place during these periods last only a few hours to a few days but can represent the largest part of the annual flow. The loads associated with these events can also account for most of the annual load. A reinforced water-quality monitoring program (especially during floods) was carried out for five years (August 2015–July 2020) on six tributaries of French Mediterranean lagoons. The loads calculated by FWMC and FDRC methods were very different. Total suspended solid loads calculated by FWMC were on average 5.0 times higher than those calculated by FDRC. Similarly, total phosphorus loads were 3.5 times higher and total nitrogen loads were 1.6 times higher. The results show that too many flood samples can lead to considerable overestimation of particulate loads calculated by the FWMC method. Dissolved nutrients, on the other hand, are much less subject to overestimation.

Per- and polyfluoroalkyl substances (PFAS) in river discharge: Modeling loads upstream and downstream of a PFAS manufacturing plant in the Cape Fear watershed, North Carolina

The Cape Fear River is an important source of drinking water in North Carolina, and many drinking water intakes in the watershed are affected by per- and polyfluoroalkyl substances (PFAS). We quantified PFAS concentrations and loads in river water upstream and downstream of a PFAS manufacturing plant that has been producing PFAS since 1980. River samples collected from September 2018 to February 2021 were analyzed for 13 PFAS at the upstream station and 43–57 PFAS downstream near Wilmington. Frequent PFAS sampling (daily to weekly) was conducted close to gauging stations (critical to load estimation), and near major drinking water intakes (relevant to human exposure). Perfluoroalkyl acids dominated upstream while fluoroethers associated with the plant made up about 47% on average of the detected PFAS downstream. Near Wilmington, Σ43PFAS concentration averaged 143 ng/L (range 40–377) and Σ43PFAS load averaged 3440 g/day (range 459–17,300), with 17–88% originating from the PFAS plant. LOADEST was a useful tool in quantifying individual and total quantified PFAS loads downstream, however, its use was limited at the upstream station where PFAS levels in the river were affected by variable inputs from a wastewater treatment plant. Long-term monitoring of PFAS concentrations is warranted, especially at the downstream station. Results suggest a slight downward trend in PFAS levels downstream, as indicated by a decrease in flow-weighted mean concentrations and the best-fitting LOADEST model. However, despite the cessation of PFAS process wastewater discharge from the plant in November 2017, and the phase-out of perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in North America, both fluoroethers and legacy PFAS continue to reach the river in significant quantities, reflecting groundwater discharge to the river and other continuing inputs. Persistence of PFAS in surface water and drinking water supplies suggests that up to 1.5 million people in the Cape Fear watershed might be exposed.

Analysis on the variation of total phosphorus concentrations and loads of inflow and outflow rivers from different water resources zones around Lake Taihu

入出湖总磷负荷变化是影响太湖湖体磷收支平衡的关键因素.基于2012-2018年水质水量监测资料，计算全湖及各水资源分区河流入出湖总磷负荷，并以水量加权计算其总磷年平均浓度，探明其时空变化规律；运用双累积曲线法分析不同分区水污关系的变化规律；以月为时间尺度，利用Pearson相关系数，揭示入湖总磷负荷分别与入湖水量、入湖总磷浓度的相关关系；通过讨论分析，明确适用于太湖的入湖总磷平均浓度计算方法、总磷负荷影响因素及控制途径.研究结果表明：（1）水量加权法比算术平均法更加适用于太湖入湖总磷平均浓度的削减管理；环太湖河流入湖总磷浓度多年平均值为0.189 mg/L，明显高于出湖的0.079 mg/L，入湖总磷负荷多年平均值为2206.51 t，也高于出湖的873.73 t；2016年丰水年入、出湖及净入湖总磷负荷均最高，分别为2993.9、1341.1、1652.8 t.（2）湖西区河流入湖总磷浓度多年平均高于所有其他分区，其值为0.226 mg/L，浙西区、武澄锡虞区河流出湖总磷浓度多年平均分别为0.114和0.109 mg/L，均高于其他分区.不同水资源分区的水污关系呈现不同的年际变化规律，但除杭嘉湖区外，其他分区均未表现出入湖总磷得到有效控制的迹象.（3）湖西区和浙西区入湖总磷年负荷多年平均值分别为1748.71和278.61 t，两区各年入湖总磷负荷占总入湖负荷的比重位居前二，湖西区的都在67.88%以上；而浙西区在7%~19%之间变化；太浦河出湖总磷年负荷多年平均值为272.27 t，自2014年起其占出湖总磷年负荷的比重均在30%以上.（4）不同水资源分区月入湖总磷负荷均与入湖水量存在较好的相关关系，且除阳澄淀泖区外，其他分区月尺度入湖总磷负荷变化主要受入湖水量控制.（5）2016年丰水年湖西区入湖河流总磷浓度较高、入湖水量大的共同作用导致了其入湖总磷负荷的显著增加.研究成果可为太湖的磷外源负荷控制提供科学依据.;The variation in inflow and outflow total phosphorus (TP) loads is a key factor affecting the balance of the phosphorus budget for Lake Taihu. Based on the monitoring data of water quality and quantity from 2012 to 2018, the TP load of the rivers in each water resources zone around Lake Taihu were calculated, the flow-weighted mean annual concentrations of TP were attained, and their spatio-temporal variations were analyzed; the double accumulation curve method was adopted to analyze the relationships between water quantity and TP load in different zones; the Pearson correlation coefficient was utilized to reveal the correlation between the TP load and water quantity as well as TP concentrations of the inflow at the monthly time scale; through discussion and analysis, the calculation method of the average TP concentration of inflow applicable to Lake Taihu, the influencing factors of TP loads and measures of controlling the TP load were investigated. The results showed that:(1) The flow-weighted mean concentration of TP is the preferred approach for calculating average concentrations of tributaries with variable flows for Lake Taihu; The average annual TP concentration of inflow rivers of Lake Taihu was 0.189 mg/L, significantly higher than that of outflow rivers (0.079 mg/L). The average annual TP load of the inflow was 2206.51 t, which was also much higher than that of the outflow (873.73 t); in the high flow year of 2016, the TP loads of the inflow, outflow and net inflow of the lake were the largest among 2012-2018, and their values were 2993.9, 1341.1 and 1652.8 t, respectively. (2) The average annual TP concentration of inflow rivers in the Huxi zone was 0.226 mg/L, which was higher than that in other zones; the average annual TP concentrations of outflow rivers in the Zhexi and Wuchengxiyu zones were 0.114 and 0.109 mg/L, which were higher than those in other zones. The interannual variation of the relationships between water quantity and TP load in different water resources zones behaved differently, suggesting that TP pollution was not effectively controlled on the whole except in the Hangjiahu zone. (3) The average annual TP load of the inflow rivers in the Huxi zone and Zhexi zone were 1748.71 and 278.61 t, respectively. The Huxi zone accounted for above 67.88% of the annual total inflow TP load, and the percentage for the Zhexi zone was between 7% and 19%; the average annual outflow TP load of Taipu River was 272.27 t and accounted for more than 30% of the total outflow TP load since 2014. (4) There were good correlations between the monthly TP load and water quantity of the inflow in different water resources zones. Moreover, except in the Yangchengdianmao zone, the variation of monthly TP load of the inflow in other zones was mainly controlled by the water quantity of the inflow. (5) In the high flow year of 2016, the combined effect of high concentration of TP and large water quantity of inflow rivers in the Huxi zone led to a significant increase in the TP load of the inflow. The research results can provide a scientific basis for external phosphorus load control for Lake Taihu.

Framework for produced water discharge management with flow-weighted mean concentration based economic model predictive control

Offshore discharges of produced water (PW) is heavily regulated by government authorities through flow-weighted mean concentration (FWMC). To cope with regulations, treatment facilities are commissioned to reduce the oil content in water. However, disturbances may affect their performance, leading to regulation infringement. Produced water re-injection (PWRI) facilities can affect the FWMC by reducing oil discharge, and increasing re-injection. This strategy can aid the offshore facility in achieving the regulation requirements. Nevertheless, if improperly done, energy consumption can significantly increase, and regulations might still be violated. Therefore, we developed a framework for an Economical Model Predictive Control (EMPC) based on FWMC. It comprises an EMPC to which FWMCs are added as constraints; and a logic algorithm to maintain the necessary conditions for feasibility of the environmental constraint. Closed-loop results show that this strategy complies with environmental regulation, and adapts discharges of PW and pumping energy consumption based on facilities’ historical data.

Then and now: Revisiting nutrient export in agricultural watersheds within southern Ontario’s lower Great Lakes basin

An enhanced understanding of nonpoint source (NPS) nutrient export to the lower Great Lakes is needed to inform land use and land management decisions within southern Ontario. However, this understanding is limited by a lack of long-term, temporally-intensive monitoring. To address this knowledge gap, we revisit six agriculturally-dominated subwatersheds in southern Ontario, which were intensively studied during the mid-1970s, to assess changes in hydrology and NPS nutrient contributions. We compared 1975–1977 to 2016–2018 stream runoff, nutrient export (kg/day∙km2), and flow-weighted mean concentrations (FWMCs) of total phosphorus (TP), total dissolved phosphorus (TDP), total nitrogen (TN), nitrates (NO3–+NO2–) and Total Kjeldahl Nitrogen (TKN). Relative to the 1970s, runoff increased at three of six watersheds (by ~20–35%) while TP and TDP export increased at five watersheds (by ~50–125%). The increases in TP and TDP FWMCs were lower relative to phosphorus export changes at the three watersheds with increased runoff, suggesting that hydrology is an important driver of phosphorus export at these sites. Interestingly, export of TN and nitrates increased while TKN export decreased at most watersheds. We further note a shift in the timing of nutrient export at most sites, with ~40–70% of export now occurring during the winter and fall seasons whereas ~40–85% of past export occurred during spring and summer. These findings support an enhanced importance of non-growing season nutrient export from agricultural watersheds since the mid-1970s and stresses the need for targeted best management practices specific to the fall and winter seasons.

Contribution of bunker silo effluent discharged via a riparian zone to watershed phosphorus loads

Nutrient losses from agricultural operations are a major contributor to the eutrophication of freshwaters. Although many studies have quantified diffuse nutrient losses, less is known about agricultural point-source contributions, such as bunker silos, to watershed phosphorus (P) loads. This study examined the contributions of a dairy farm bunker silo effluent to watershed soluble reactive P (SRP) and total P (TP) losses. The bunker silo effluent discharged to an adjacent stream via a riparian soakaway for ca. 15 years. Prior to the annual refilling of the bunker silo, flow weighted mean concentrations of SRP (TP) were similar between stream locations up and downstream of the farm. After the bunker silo was refilled, flow-weighted SRP (TP) concentrations in the stream increased by factors of 1.5(2.2) during events and 3.1(2.3) during baseflow. Higher P concentrations occurred in the riparian soils receiving bunker silo effluent (525–3125 mg/kg TP, and 0.1–9.9 mg/kg water extractable P (WEP), compared with 525–939 mg/kg TP, and 0.11–1.43 mg/kg WEP on the opposite side of the stream with no bunker silo effluent. Riparian soils impacted by the bunker silo were near P-saturation, and the riparian zone did little to reduce P transfer in shallow groundwater. The net contributions of bunker silo effluent to annual watershed P losses were 32% (SRP) and 22% (TP). This study highlights the importance of agricultural point sources, and the need to quantify their contributions to watershed P budgets to target P remediation effectively.

Long-term effect of poultry litter application on phosphorus balances and runoff losses.

Assessment of annual and cumulative impacts of phosphorus (P) management strategies at field and watershed scales is needed to improve crop use efficiency and minimize environmental impacts. The objectives of this study were (a) to assess relationships among P balance, soil test P (STP) concentration, and runoff dissolved reactive P (DRP) concentration from fields receiving different poultry litter application rates (0.0-13.4 Mg ha-1 ) and (b) to determine the effect of long-term poultry litter application to fields on watershed DRP loss. Nutrient management practices, crop yield, STP, and runoff losses were assessed from nine fields and two watersheds located near Riesel, TX, from 2000 to 2015. Field-scale P balances were largely controlled by P application rate and exhibited a positive relationship with STP and runoff DRP flow-weighted mean concentration. Using a before-after control-impact experimental design that included monitoring at both field and watershed scales showed the influence of field P management on watershed DRP loss varied according to both source (i.e., P application rate, impacted area) and transport (i.e., hydrological connectivity) factors. Increased risk of watershed DRP loss was observed during wet years and years with two poultry litter applications to fields within the watershed. The percentage of the total watershed area receiving high rates of poultry litter also played a critical role in determining the risk of DRP loss. Findings highlight the impact of long-term P management strategies on DRP loss at both field and watershed scales and show the importance of incorporating hydrologic connectivity when assessing conservation effects on water quality.

Influence of feedlot manure on reactive phosphorus in rainfall runoff during the transition from continuous to legacy phases

Little research has compared land application of stockpiled (SM) or composted (CM) beef feedlot manure with straw (ST) or wood-chip (WD) bedding on loss of reactive phosphorus (RP) in runoff. We conducted a 6 yr (2013–2018) rainfall simulation-runoff study and utilized surface (0–5 cm) soil collected from a long-term (since 1998) field experiment on a clay loam soil in southern Alberta, Canada. The treatments consisted of SM or CM with ST or WD bedding applied at 13, 39, and 77 Mg·ha−1 (dry weight), as well as an unamended control and mineral fertilizer treatment. Surface soil was collected from all treatments after 15–17 (C15, C16, C17; 2013–2015) continual annual applications, and then 1–3 yr (L1–L3, 2016–2018) into the legacy phase after manure applications were first discontinued in 2015. The soil was packed into runoff trays, and flow-weighted mean concentrations (FWMCs) and mass loads of RP5 (&lt;5 μm filter) in runoff water were determined during rainfall simulations. Our findings generally supported our null hypothesis of similar RP5 losses for manure type (CM = SM) and bedding (ST = WD) for most years. Successively higher application rates increased RP5 loss by 32%–121%. Termination of long-term applications dramatically reduced FWMCs by 58%–77% and mass loss by 56%–65% from the C17 to L3 years. This suggests an accumulation of soil P during continuous phase and depletion during legacy phase; therefore, lower application rates or termination of applications may reduce RP5 loss in runoff.

Stratified Soil Sampling Improves Predictions of P Concentration in Surface Runoff and Tile Discharge

Phosphorus (P) stratification in agricultural soils has been proposed to increase the risk of P loss to surface waters. Stratified soil sampling that assesses soil test P (STP) in a shallow soil horizon may improve predictions of P concentrations in surface and subsurface discharge compared to single depth agronomic soil sampling. However, the utility of stratified sampling efforts for enhancing understanding of environmental P losses remains uncertain. In this study, we examined the potential benefit of integrating stratified sampling into existing agronomic soil testing efforts for predicting P concentrations in discharge from 39 crop fields in NW Ohio, USA. Edge-of-field (EoF) dissolved reactive P (DRP) and total P (TP) flow-weighted mean concentrations in surface runoff and tile drainage were positively related to soil test P (STP) measured in both the agronomic sampling depth (0–20 cm) and shallow sampling depth (0–5 cm). Tile and surface DRP and TP were more closely related to shallow depth STP than agronomic STP, as indicated by regression models with greater coefficients of determination (R2) and lesser root-mean square errors (RMSE). A multiple regression model including the agronomic STP and P stratification ratio (Pstrat) provided the best model fit for DRP in surface runoff and tile drainage and TP in tile drainage. Additionally, STP often varied significantly between soil sampling events at individual sites and these differences were only partially explained by management practices, highlighting the challenge of assessing STP at the field scale. Overall, the linkages between shallow STP and P transport persisted over time across agricultural fields and incorporating stratified soil sampling approaches showed potential for improving predictions of P concentrations in surface runoff and tile drainage.

Long-term regional nutrient contributions and in-lake water quality trends for Lake Okeechobee

Faridmarandi S, Khare YP, Naja GM. 2020. Long-term regional nutrient contributions and in-lake water quality trends for Lake Okeechobee. Lake Reserv Manage. XX:XXX–XXX. A long-term dataset was used to investigate the historical regional changes in Lake Okeechobee water quality from water year 1974 to 2018. During this period, the total phosphorus (TP) load from the northern region to the lake remained statistically stable while the in-lake TP concentrations showed a statistically significant increasing trend, despite decades of state plans to restore this ecosystem. During the investigated period of record, the inflow TP flow weighted mean concentration from the southern region had a slightly higher value of 0.191 mg/L (when compared to the northern region) with a statistically significant increasing trend. The average annual total nitrogen unit area inflow loading (678.6 kg/km2) and flow weighted mean concentration (4.35 mg/L) into Lake Okeechobee from the southern region had the highest levels when compared to other regions. Lake Okeechobee TP sedimentation rate showed a statistically significant decreasing trend indicating a reduction in the capacity of the lake sediments to assimilate phosphorus. Results showed that the lake sediments may become a continuous source of phosphorus in water year 2035. Sustained and long-term efforts on the scale of decades will be required for restoring the lake using a multipronged approach consisting of mainly the implementation of nutrient/fertilizer best management practices and the construction of ∼200 km2 of constructed wetlands for a total project cost of US$4.26 billion. Unless a proven cost-effective technology is developed to tackle the legacy TP problem in the lake sediments, a prioritization of state resources is required while focusing on Lake Okeechobee watershed.

Influence of feedlot manure amendments on dissolved organic carbon in runoff during transition from continuous to legacy applications

Little research has compared land application of stockpiled (SM) or composted (CM) beef feedlot manure with straw (ST) or wood-chip (WD) bedding on loss of dissolved organic carbon (DOC) in runoff. We conducted a 6 yr (2013–2018) rainfall simulation-runoff study and utilized surface (0–5 cm) soil collected from a long-term (since 1998) field experiment on a clay loam soil in southern Alberta, Canada. The treatments consisted of SM or CM with ST or WD bedding applied at 13, 39, and 77 Mg·ha−1 (dry weight), as well as an unamended control and mineral fertilizer treatment. Surface soil was collected from all treatments after 15–17 (C15, C16, and C17; 2013–2015) continual annual applications and then after one to three legacy years (L1–L3, 2016–2018) after manure applications were first discontinued in 2015. The soil was packed into runoff trays, and flow-weighted mean concentrations (FWMCs) and mass loads of DOC in runoff water were determined during rainfall simulations. Mean DOC losses were generally significantly (P ≤ 0.05) lower for CM with ST bedding compared with the other manure type – bedding treatments in certain years and were consistent with this amendment having the lowest total carbon (C) content. The total C content of the amendments explained 92% of the variation in DOC concentration. Termination of long-term manure applications reduced FWMCs by 85%–91% and mass loss by 76%–89% from the C17 to L3 year. Therefore, our findings suggested that composting manure with ST or discontinued long-term manure application may reduce DOC loss in runoff.

Closed depressions and soil phosphorus influence subsurface phosphorus losses in a tile-drained field in Illinois.

Artificial subsurface (tile) drainage systems can convey phosphorus (P) from agricultural fields to surface waters; however, controls of subsurface dissolved reactive P (DRP) losses at the sub-field scale are not fully understood. We characterized subsurface DRP loads and flow-weighted mean concentration (FWMC) from January 2015 through September 2017 to determine seasonal (growing vs. non-growing) patterns from 36 individually monitored plots across a farm under a corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation in east-central Illinois. Using linear mixed models, we investigated the effects of soil test P (STP), depression depth, and their interaction with precipitation and P fertilization on subsurface DRP losses. Dissolved reactive P loads in drainage tiles increased with precipitation and were greatest during the non-growing season (NGS) in 2016 and 2017. Annual subsurface DRP loads were positively related to STP, and during the NGS, there was a positive relationship between depression depth quantified at the plot-scale and subsurface DRP loads and FWMC. Along a depression-depth gradient, piecewise regression displayed a threshold at a depth of 0.38m at which STP increased, indicating soil P accumulation in deeper closed depressions. Our study highlights the need to identify areas with the greatest risk of subsurface P losses to implement sub-field scale nutrient management practices.

An analysis of the sample size requirements for acceptable statistical power in water quality monitoring for improvement detection

Many water quality managers seek to demonstrate reductions in pollutants after a remedial program or policy change of some sort is implemented, but there is little information in the literature to help guide the extent of water quality sampling that is required to be confident that a change has occurred. Statistical power refers to the likelihood of avoiding a Type II error in hypothesis testing. It is critical to examine statistical power levels to ensure results are not unduly influenced by insufficient quantity of data. This study presents the first published record, to the best of our knowledge, on sample size requirements to achieve acceptable levels of statistical power in hypothesis testing of annual water quality (nutrients) in streams. We examined 13 temperate agricultural watersheds spanning a gradient of size from 11 to 16,000 km2 using data synthesized from long-term flow and water quality records. We found that achieving commonly accepted levels of statistical power (0.8) after reductions of 20% in load or flow-weighted mean concentration (FWMC) required an inordinate quantity of data (50–250 years for load, 10–120 years for FWMC), while achieving statistical power of 0.8 after reductions of 80% of load or FWMC required very little data (2–4 years for FWMC, 2–7 years for load). Load reductions of 40% required a range of 8–50 years of data depending on analyte, while FWMC reductions of 40% required 3–10 years of total phosphorus (TP) data, 5–25 years for soluble reactive phosphorus (SRP), and 2–6 years for nitrate (NO3). We examined relationships among times to achieve statistical power and a number of common landscape descriptors (discharge, baseflow index, basin size, concentration-discharge slope) and found no discernable relationships for either TP or SRP, whereas catchments with higher baseflow indices were found to have lower data requirements for achieving statistical power of 0.8 for NO3. We also show through subsampling experiments that higher frequency sampling tended to reduce data requirements to achieve acceptable statistical power, though these gains diminish as the sample frequency increases. The information presented will help those tasked with watershed monitoring to design appropriate sampling regimes to ensure adequate data are obtained to detect change.

Wet season nitrogen export from a residential stormwater pond.

Stormwater runoff is recognized as a cause of water quality degradation because it may carry nitrogen (N) and other pollutants to aquatic ecosystems. Stormwater ponds are a stormwater control measure often used to manage stormwater runoff by holding a permanent pool of water, which reduces the peak flow, magnitude of runoff volume, and concentrations of nutrients and pollutants. We instrumented the outlet of a stormwater pond in an urban residential neighbourhood in Florida, United States to (1) investigate the concentration and composition of N forms during the summer rainy season (May to September 2016), and (2) determine the bioavailability of organic N in the stormwater pond with a bioassay experiment. A total of 144 outflow water samples over 13 storm events were collected at the outlet of the stormwater pond that collects runoff from the residential catchment. Samples were analysed for various inorganic N [ammonium (NH4–N), nitrate (NO3–N)], and organic N forms [dissolved organic nitrogen (DON), and particulate organic nitrogen (PON)]. Flow-weighted mean concentration of total N (TN) in pond outflow for all collected storm events was 1.3±1.42 mg L-1, with DON as the dominant form (78%), followed by PON and NO3–N (each at 8%), and NH4–N (6%). In the bioassay experiment, organic N (DON+PON) was significantly decreased by 25–28% after 5 days of incubation, suggesting that a portion of the DON carried from the pond outflow to receiving water bodies may be bioavailable. These results suggest that efforts to mitigate stormwater N outflows from urban ponds should incorporate both inorganic and organic N in management plans.