Constructed treatment wetlands are efficient at retaining a range of pesticides, however the ultimate fate of many of these compound is not well understood. This study evaluated the effect of drain-fill cycling on the mineralization of chlorpyrifos, a commonly used organophosphate insecticide, in wetland sediment–water microcosms. Monitoring of the fate of 14C ring-labeled chlorpyrifos showed that drain-fill cycling resulted in significantly lower mineralization rates relative to permanently flooded conditions. The reduction in mineralization was linked to enhanced partitioning of the pesticide to the sediment phase, which could potentially inhibit chlorpyrifos hydrolysis and mineralization. Over the nearly two-month experiment, less than 2.5% of the added compound was mineralized. While rates of mineralization in this experiment were higher than those reported for other soils and sediments, their low magnitude underscores how persistent chlorpyrifos and its metabolites are in aquatic environments, and suggests that management strategies and ecological risk assessment should focus more on ultimate mineralization rather than the simple disappearance of the parent compound.

Transportation activities generate a wide range of pollutants, including particulates, copper (Cu), zinc (Zn), and lead (Pb), that accumulate on roadway surfaces. Stormwater runoff can flush these species to, and impair the hydrology of, receiving waters. This study evaluated the ability of roadside infiltration, a management strategy that relies on natural dispersion and permeation of runoff through roadside soils, to mitigate the impacts of roadway runoff in semiarid eastern Washington. Monitoring of nine runoff events totaling 60 mm of runoff showed complete infiltration within 2 m of the roadway edge. Levels of Cu, Zn, and Pb in the aqueous phase of runoff ranged from 10–200 mg=L, while levels in sediments ranged from 20–75 mg=kg (dry weight). Two-thirds of Cu and Zn in runoff were associated with the aqueous phase, while two-thirds of Pb was associated with sediment. The midsized fraction (0.5–2.0 mm) of sediment in runoff contained the greatest mass of metals (around 40%). While Cu and Zn in roadside soils were slightly higher at the roadway edge, all concentrations were near or below background levels. In contrast, Pb concentrations increased with depth and distance from the roadway, likely the result of historical deposition and=or Pb leaching from deicing salt applications. From a management perspective, results suggest that: (1) roadside infiltration is a suitable method to manage the environmental impacts of roadway stormwater runoff in semiarid climates, and (2) the midsized fraction of roadway sediments should be targeted for capture because they contain the greatest mass of metals.

An innovative method of reducing external phosphorus (P) loading to lakes uses engineered systems to treat lake inflows with aluminum sulfate (alum). In this study we used a series of jar tests to examine the optimal alum dose and mixing regime to remove P from Matthiesen Creek, an important external source of P to Jameson Lake. Matthiesen Creek is a good candidate for alum treatment because the creek runs year round, and the majority of P in the spring-feed creek is in the form of bioavailable dissolved P that can be efficiently captured in alum floc. The mixing regimes in this study mimicked a range of possible treatment scenarios that relied on natural turbulence in the creek or conventional mechanical mixing, and presumed the discharge of alum floc either directly to the lake or to an on-shore settling basin. Jar tests showed that an alum dose of 5 mg-Al/L was sufficient to decrease P from around 0.13 mg-P/L to below 0.02 mg-P/L for most mixing regimes. For all mixing regimes, doses of up to 20 mg-Al/L did not depress pH below the recommended minimum pH of 6. Flash mixing prior to low-intensity mixing did not enhance P removal over low-intensity mixing alone, but flash mixing alone resulted in lower levels of P removal from creek water. Jar testing with a mixture of alum-treated creek water and lake water showed that lake waters tended to inhibit P uptake by alum floc. This, combined with the fact that high pH favors the formation of the aluminate ion which could exhibit chronic toxicity to aquatic biota, suggests that discharge of alum solids directly to the lake should be avoided. We recommend an engineered inflow treatment system on Matthiesen Creek that maintains an alum dose of 5–10 mg-Al/L under moderate mixing conditions (Gt of 1,000–3,000) with alum floc collected in an on-shore settling basin.

Constructed treatment wetlands (CTWs) have been used effectively to treat a range of wastewaters and non-point sources contaminated with nitrogen (N). But documented long-term case studies of CTWs treating dilute nitrate-dominated agricultural runoff are limited. This study presents an analysis of four years of water quality data for a 1.6-ha surface-flow CTW treating irrigation return flows in Yakima Basin in central Washington. The CTW consisted of a sedimentation basin followed by two surface-flow wetlands in parallel, each with three cells. Inflow typically contained 1–3 mg-N/L nitrate and <0.4 mg-N/L total Kjeldahl N (TKN). Hydraulic loading was fairly constant, ranging from around 125 cm/d in the sedimentation basin to 12 cm/d in the treatment wetlands. Concentration removal efficiencies for nitrate averaged 34% in the sedimentation basin and 90–93% in the treatment wetlands. Total N removal efficiencies averaged 21% and 57–63% in the sedimentation basin and treatment wetlands, respectively. Area-based first-order removal rate constants for nitrate in the wetlands averaged 142–149 m/yr. Areal removal rates for nitrate in treatment wetlands averaged 139–146 mg-N/m2/d. Outflow from the CTW typically contained <0.1 mgN/L nitrate and <0.6 mg-N/L TKN. Rates of nitrate loss in wetlands were highly seasonal, generally peaking in the summer months (June–August). Nitrate loss rates also correlated significantly with water temperature (positively) and dissolved oxygen (negatively). Based on the modified Arrhenius relationship, for nitrate loss in the wetlands was 1.05–1.09. The CTW also significantly affected temperature and dissolved oxygen concentration in waters flowing through the system. On average, the sedimentation basin caused an increase in temperature (+1.7 ◦C) and dissolved oxygen (+1.5 mg/L); in contrast the wetlands caused a decrease in temperature (-1.6 ◦C) and dissolved oxygen (-5.0 mg/L). Results show that CTWs with surface-flow wetlands can be extremely effective at polishing dilute non-point sources, particularly in semi-arid environments where warm temperatures and low oxygen levels in treatment wetland water promote biological denitrification.

While constructed treatment wetlands are very efficient at polishing nitrate from secondary effluent, they are much less effective at removing ammonia. A key factor that limits ammonia oxidation via biological nitrification in vegetated wetlands is low levels of dissolved oxygen. This study evaluated the effectiveness of side-stream oxygenation to enhance ammonia removal in replicate surface-flow experimental mesocosms containing wetland sediment and plants Typha spp. Mesocosms had a water volume of 29.5 L, a hydraulic retention time of 5 days, and a hydraulic loading rate of 4.3 cm/d, and were loaded with synthetic secondary effluent contain 10 mg-N/L of ammonia. Relative to nonoxygenated controls, oxygenation increased ammonia removal rates by an order of magnitude. Areal removal rates increased from 40 mg-N/m2/d to 450 mg-N/m2/d, concentration removal efficiency increased from 10 to 95%, and area-based first-order removal rates increased from 2 m/year to 50–75 m/year. Ammonia removal rates in oxygenated mesocosms were 2- to 4-fold higher than rates reported for full-scale constructed wetlands treating secondary effluent. Results show that oxygen-activated nitrification wetlands, a hybrid of conventional oxygenation technology and wetland ecotechnology, hold promise in economically enhancing rates of ammonia removal and shrinking the wetland area needed to polish ammonia-dominated secondary effluent. Further study is needed to confirm that oxygenation can promote high rates of ammonia removal at the field scale.

Concentrations of key nutrients and metals in water overlaying profundal sediments were evaluated in replicate experimental chambers containing undisturbed sediment–water interface samples from Deer Lake, an oligo-mesotrophic lake in eastern Washington. Chambers were incubated under three sequential phases: aerobic (21 d), anaerobic (27 d), and second aerobic (14 d). In general, nutrients and metals in chamber water were lower under aerobic versus anaerobic conditions. However, in some cases compounds anticipated to appear only under anaerobic conditions, including ammonia, phosphate, and manganese, were observed during aerobic conditions. Correlation analysis elucidated a number of interactions between compounds. Phosphate correlated significantly (p<0.05) with iron during all incubation phases, suggesting that phosphorus cycling was controlled by iron. Cycling of nitrate and ammonia was tightly and significantly coupled under aerobic conditions. During both aerobic phases, nitrate increased while ammonia decreased, likely as a result of biological nitrification. Finally, mercury appeared during mildly reducing conditions and showed a significant correlation with manganese during the anaerobic phase, suggesting that Mn oxide reduction was the source of Hg in chamber water.

There is renewed interest in the use of nitrate to treat the profundal zone of lakes to inhibit anaerobic biogeochemical processes that result in the degradation of bottom water quality (e.g., sediment phosphorus release, mercury methylation). In this study we used experimental sediment–water interface chambers to quantify the rate of sediment nitrate uptake (SNU) in profundal sediments from Lake Perris, a eutrophic raw water reservoir in Southern California. Deoxygenated chamber water was spiked with nitrate, and nitrate concentration was monitored over time under quiescent conditions, followed by mixed conditions with average water velocities of 1 cm/s. Key findings included: (1) SNU decreased with decreasing nitrate concentration, (2) SNU was higher under mixed versus quiescent conditions by nearly 50%, and (3) nitrate uptake as a function of nitrate concentration followed a conventional sediment oxygen demand model in which nitrate uptake was proportional to the square root of nitrate concentration. The probable mechanism for elevated SNU under mixed conditions was an increased diffusional concentration gradient combined with a decrease in the diffusional boundary layer at the sediment–water interface, both of which enhanced the flux of nitrate from overlaying water into sediment. Managers planning to implement lake nitrate addition should account for induced nitrate demand when determining dosing rates. For example, based on our modeling efforts from this data set, SNU in Lake Perris could range by an order of magnitude, from around 12 mg N/m2 /d under quiescent, low nitrate conditions (0.1 mg N/l) to around 120 mg N/m2 /d under mixed, high nitrate conditions (5 mg N/l).

Lake Mathews is a large, oligo-mesotrophic reservoir located in Southern California. The reservoir has elevated levels of nitrate and periodically experiences hypolimnetic anoxia. Experimental sediment-water chamber incubations and reservoir water quality monitoring were conducted to evaluate how oxygen and nitrate in overlaying water affect nutrient release from profundal sediments and internal nutrient loading. In experimental incubations, under nitrate-free anoxic conditions, sediment nutrient release rates were 3.4 ± 0.8 mg-P/m2 ·d for phosphate and 2.8 ± 1.2 mg-N/m2 ·d for ammonia (average ± standard deviation; n = 6). Oxygen repressed phosphate release and greatly diminished ammonia release from sediments in experimental incubations while nitrate only repressed phosphate release. Similar nutrient release dynamics were observed in the reservoir. Nutrient release rates estimated from seasonal nutrient profiles collected from the reservoir were 3.4 mg-P/m2 ·d for phosphate and 2.5 mg-N/m2 ·d for ammonia. Ammonia accumulation in the hypolimnion commenced with the onset of anoxic conditions, but phosphate accumulation did not start until nitrate disappeared from bottom waters approximately 6 weeks later. The time lag decreased total internal phosphorus loading by approximately 25% relative to hypothetical nitrate-free conditions. Laboratory and field data show that both oxygen and nitrate repress sediment phosphate release, likely via the maintenance of an oxidized surficial sediment layer that retains phosphate in iron-oxide complexes. However, only oxygen and not nitrate was effective in decreasing sediment ammonia release, likely by enhancing biological nitrification and assimilation in surficial sediments under oxic conditions. A number of in-lake management strategies have been developed to inhibit internal nutrient loading including calcium nitrate addition, aluminum sulfate addition, and oxygenation. In our view, the deliberate addition of nitrate to lakes and reservoirs poses several risks that must be carefully considered when evaluating strategies to control sediment phosphorus release.

Nitrate removal rates and dissolved oxygen (DO) levels were evaluated in small batch-mode wetland mesocosms with two different plant species, cattail (Typha spp.) and bulrush (Scirpus spp.), and associated mineral-dominated sediment collected from a mature treatment wetland. Nitrate loss in both cattail and bulrush mesocosms was first-order in nature. First-order volumetric rate constants (kV) were 0.30/d for cattail and 0.21/d for bulrush and rates of nitrate loss were significantly different between plant treatments (p < 0.005). On an areal basis, maximum rates of nitrate removal were around 500 mg N/(m2-d) early in the experiment when nitrate levels were high (> 15 mg N/L). Areal removal rates were on average 25% higher in cattail versus bulrush mesocosms. DO in mesocosm water was significantly higher in bulrush versus cattail (p < 0.001). DO in bulrush generally ranged between 0.5 and 2 mg/L, while DO in cattail mesocosms was consistently below 0.3 mg/L. Based on cumulative frequency analysis, DO exceeded 1 mg/L around 50% of the time in bulrush, but only 2% of the time in cattail. DO in bulrush exhibited a statistically significant diel cycle with DO peaks in the late afternoon and DO minimums in the early morning hours. Difference in nitrate removal rates between wetland plant treatments may have been due to differing plant carbon quality. Cattail litter, which has been shown in other studies to exhibit superior biodegradability, may have enhanced biological denitrification by fueling heterotrophic microbial activity, which in turn may have depressed DO levels, a prerequisite for denitrification. Our results show that the cattail is more effective than bulrush for treating nitrate-dominant wastewaters.

Hypolimnetic oxygenation can improve water quality by decreasing hypolimnetic accumulation of reduced compounds that complicate potable water treatment. Historically, aeration systems have been undersized because designers have not accounted for increases in sediment oxygen demand (SOD) resulting from the operation of aeration systems. A comprehensive study was performed to estimate the hypolimnetic oxygen demand (HOD) in San Vicente Reservoir, a eutrophic raw water reservoir in San Diego. Chamber experiments confirmed that turbulence and oxygen concentration at the sediment-water interface dramatically affected SOD. Values ranged from under 0.2 g/m2/day under quiescent low-oxygen conditions to over 1.0 g/m2/day under turbulent high-oxygen conditions. Based on a statistical evaluation of historical oxygen concentrations in the reservoir and anticipated increases in SOD resulting from operation of an oxygenation system, a design HOD of 16,400 kg/day was estimated. This is approximately four times the HOD observed in the spring after the onset of thermal stratification. Laboratory chamber experiments confirmed that maintenance of a well-oxygenated sediment-water interface inhibited the release of phosphate, ammonia, iron, and manganese from sediments. In addition, hydrodynamic modeling using DYRESM-WQ showed that operation of a linear diffuser oxygenation system would not significantly affect thermal stratification.