My talk today is about using weirs as a conservation practice to manage sediment and phosphorus pollution.

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The Mississippi River watershed drains half the continental United States.Its rich river valley soils make for very productive agriculture.

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•Excess nutrient inputs in the form of non-point source pollution from agriculture travel through the watershed until they are finally deposited in the Gulf of Mexico where they contribute to a hypoxic “dead zone” the size of Connecticut. •Not only are these nutrients problematic on a large, continental scale, but also contribute to eutrophication locally and regionally in headwaters and smaller drainage basins as they make their way to the coast.

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•Eutrophication is a natural process when it occurs over the course of centuries.•Increases of anthropogenic nutrient inputs accelerate this process instead over decades, leading to habitat degradation.

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•The nutrients set off a cascading effect. One of the leading limiting nutrients in aquatic systems is phosphorus. •Excess phosphorus leads to an increase in primary producer biomass. When these organisms die, the decomposition by respiring microorganisms deplete oxygen levels leading to species migration or die off.

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•One of the challenges in managing P is that there is no significant gaseous phase in it’s cycle. Once it enters a system, it stays until further runoff or erosion carry it to another system downstream.•Plant and microorganism assimilation is a temporary solution, as biological uptake only sequesters P for the life of the tissue. Death or senescence releases P back into the system.•Adsorption to mineral surfaces or precipitation with ions in soils and sediment burial are the only long term solutions to P storage.•My research focuses on this portion and the abiotic factors that determine the primary direction of these arrows.

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•Most P in the overlying water column is particulate P, and particulate P settles out of the water, depositing onto sediments. Consequently, overall net flux of P is out of the water column and into soils. •So, what’s the problem?•Continuous accretion increases the dissolved P concentrations in soil pore water. Greater concentrations of P in soil pore water than the overlying water column means more dissolved, bioavailable P, moving out of soil or sediment and into the overlying water column. Because P is such a limiting resource, this can greatly impact aquatic ecosystems.•The interplay in equilibrium processes of P movement into and out of the water column determines sediment’s ability to behave like P sources or sinks.•This ability in turn, is regulated indirectly on abiotic factors such as redox potential and pH.•The reduction of ferric (Fe(III)) iron in FePO4 to ferrous (Fe(II)) iron causes PO43+ to become soluble.•P solubility is pH dependent because of the effect pH has on iron and aluminum phosphate precipitation at low pH and calcium phosphate precipitation at high pH.•Manipulation of hydraulic residence time affects both redox and pH, and therefore indirectly effects P retention.

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•Controlled drainage in ag ditches using weirs offer a potential solution•They reduce water velocity, thereby slowing the transport of sed and p, while maintaining the original drainage function during storm events, as shown in the inset

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•Due to the new application of weirs installed in agricultural drainage ditches, little is known regarding how they change the system over time.•The presence of a weir can affect sediment accumulation as well as water depth, which in turn can affect abiotic factors such as pH•These changes in abiotic factors then indirectly relate to how weirs affect inorganic p retention by ditch soils

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•Here is an illustration of how weirs increase hydraulic residence time•Clearly there is a marked difference in water level above and below the weir. (arrows denote flow direction)•The other thing I’d like to point out is the proximity of an oxbow lake. This weir is the only thing intercepting runoff from the surrounding ag fields before reaching this primary water body.

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•In order to simulate, isolate, and test, under controlled conditions how increased hydraulic residence time potentially alters abiotic factors and p behavior, I conducted a laboratory experiment in microcosm chambers•Ditch seds were collected from 16 sites along the lower Mississippi river alluvial valley•Three agricultural drainage ditches were sampled from each site (total n = 48). Sediment was collected from three locations along each ditch and homogenized within the microchamber

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•Ditch seds were treated with 3 days, 3 weeks, or 3 months of inundation with deionized water.•Sub samples of sediments were withheld from any inundation and regarded as “no inundation treatment” for comparison. •pH was measured throughout the treatment•At the end of the appropriate treatment duration, a platinum electrode that had been in place for the length of the experiment, to allow for equilibration, recorded redox potential (Eh) of sediments to determine anaerobic severity, and sediment samples were collected.

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•The collected sediment samples were then fractionated for phosphorus. •The first step extracted soluble and loosely bound phosphorus, the second, aluminum phosphate, third, iron phosphate, and finally reductant-soluble phosphorus.

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•The order of operations for my statistics started with a Shapiro-Wilk normality test.•If the data was normally distributed, a Levene Homogeneity of variance test was run•A oneway ANOVA was run on data with equal variance, and a Welch ANOVA was used in the case of unequal variance.•Tukey- Kramer HSD (honestly significant difference) means comparisons test was used to determine which means were significantly different.•If data were not normally distributed, a Kruskal wallis rank sums test was used•Wilcoxon pairwise comparison test was then used to determine which treatment means differed•In my results, normal data are shown with an F test statistic and non normal data are shown with a χ2 test statistic.•For all analysis, alpha equals .05

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As expected, longer inundation meant greater pH. Again, availability of P depends indirectly on pH, and at lower pH forms non-bioavailable compounds with iron and aluminum oxides and hydroxides through specific adsorption. Conversely, P is most bioavailable in a more neutral pH range of 6 to 7.

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Also expectedly, longer inundation led to a decrease in Eh. In aerobic soils, phosphate precipitates as iron(III) phosphates. Under inundated, anaerobic conditions (ranging from 300-100 mV), ferric iron in FePO4 is reduced to soluble ferrous iron causing PO4

3- to become soluble.

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•Sediments in no inundation treatments had more acidic pH and were aerobic, which was reflected by less soluble and loosely bound P and greater Al, Fe and reductant-soluble P. •Traditionally, agricultural drainage ditches are ephemeral systems and would be represented by the three day inundation treatment. As expected, after brief inundation sediments became more anaerobic and pH rose. This change corresponded with a rise in soluble P, and a drop in Al, Fe, and reductant-soluble P. •Average sediment pH was even greater and Eh lowest in 3 month inundated sediments, however, Fe and reductant-soluble P concentrations were as high as no inundation sediments. In fact, FePO4 concentrations were back to no inundation levels after only three weeks of inundation. •In this study, under conditions where drainage ditch sediments were quickly inundated and drained, they displayed characteristics of behaving like P sources and would be detrimental in the pursuit for P reduction to downstream waters. •In ditches equipped with weirs, sediments would be inundated longer and more consistently. With weirs holding a steady hydraulic residence time, they could influence sediments to act as P sinks even if they alter environmental conditions known to release P and can therefore be utilized as an effective nutrient management tool.

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•In order to cross reference experimental data with field conditions and record weir affects on sed deposition rates I conducted two field studies•The first field study monitored four locations in the east ditch of Stovall Farms associated with Harris Bayou starting at time of weir installation•The locations included 3 weirs and one inflow site for comparison.

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Weirs were built with limestone rip rap along a two stage ditch.A two stage ditch comprises a main channel along with a second stage step within the full ditch width.

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Intervals of locations were placed at a distance so as to avoid influences from other weirs. The position of another study location occurs past the point where weir height intersects slope

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•Sediment and water depth were recorded each month for a year using rebar as a permanent reference marker.•Sediment samples were collected monthly for six months.

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•Sediment samples were fractionated for P with the same method, but for purposes of comparison, they were then classified as potentially bioavailable (soluble and loosely bound P, and FePO4) and non-bioavailable (AlPO4, and reductant-soluble P).

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Potentially bioavailable and non-bioavailable concentrations were then translated into bioavailability ratios with large ratio numbers representing more potentially bioavailable P and less non-bioavailable P. Conversely, small ratio numbers represent less potentially bioavailable P and more non-bioavailable P. Therefore, in the interest of nutrient reduction management, smaller ratio numbers are preferred in drainage ditch sediments.

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•The same order of operations for statistics was preformed. •However, with this study, all data sets were not normal.•With this being a field study, and therefore many uncontrollable variables being the nature of field collected data, alpha was set to .1•Bivariate least squares linear regression analysis was used to determine if any variables were correlated with time at each site.

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Just visually, here are the changes that can occur from one month after installation in January to 7 months later in August.

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•Average water depth was found to be significantly greater behind weirs, justifying the laboratory experiment, simulating weir effects on inundation of sediments.

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•There was no significant difference in sediment depth among locations.•This study monitored sediment depth at one permanent reference marker in the center of the channel behind the weir. Sediment could be depositing on the second stage shoulders of the two stage ditch design, or within the recesses of the rock rip rap, since these weirs were not built with a solid core (such as an earthen berm). Future studies may want to incorporate additional sediment measurement reference points at varying distances from the weir and ditch banks to create a more accurate picture of weir contribution to sediment dynamics.

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Average pHs behind all weir locations were significantly greater than at the inflow, and also fell within the greatest P solubility pH range.

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Looking at the P fractions alone, no decipherable pattern emerges, other than there being no significant difference among locations of the two bioavaialbility extremes.

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Despite significant differences in inundation and pH, there is no significant difference in bioavailability ratio among locations.

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•Not only did the inflow have the lowest average water depth, but it also had declining water depth over time.•Also, although presence of a weir did lead to greater average pH, the inflow was the only location where pH was correlated with time. •And while there was no significant difference in the bioavailability ratio averages, the inflow, remaining in a sate of inconsistent inundation, was the only location that revealed a rising bioavailability ratio in the first six months of observations. •By converting inconsistently inundated sediments into more reliably saturated sediments, weirs had no variables correlated with time. •These results suggest weirs temper the traditional ephemeral nature of agricultural drainage ditch sediments, prolonging inundation times and allowing P to interact with sediments under equilibrated conditions. • But what about the long term effects of weirs?

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•If weirs are expected to become an established conservation practice, it is important to know how they perform after the first year of installation.•The second field study measured 1, 2, and 4 year old weirs in order to compare differences across weirs of varying maturity•Because the application of weirs in agricultural drainage ditches is so new, I worked with very small sample sizes.

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The weirs were located adjacent to two oxbow lakes, Bee Lake and Wolf Lake.

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•All weirs were built with limestone rip rap•This was about the extent of their similarity.•Due to differences in weir heights, ditch lengths and widths, and acreages drained comparisons across systems cannot be done. •But this study seeks to compare only the variable of age.•So, to eliminate these other variables, all data was collected during non-storm event conditions in the same month of August, and each weir was paired with a respective control.

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•Controls were located within the same drainage ditch as their respective weirs so as to eliminate undesired variables, but also located far enough away so as to be outside the influence of the weir. •Field measurements were recorded and samples were collected directly up stream of weir sites and control sites. •Results from the control were subtracted from results from the weir. •This way other variables are relativized, and relevant comparisons can be made among weirs based solely on age.

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•Due to these weirs being installed before the start of this study, there were no permanent reference markers in place to measure sediment. •Three techniques were used•First, deposition layers in soil cores were determined with visual and tactile differences from original ditch bottom soil

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Second, the differences between sediment height above and below the weir were determined using a laser held level at the top of the weir and shot to a marker held above and below the weir. The height from the top of the sediment to the laser mark was recorded. The sediment depth is determined by the difference between the heights

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•Third, a metal meter stick was pushed through the deposition layer until reaching compacted original ditch bottom. •This was the most reliable technique due to vegetation and water depth constraints. •Where conditions allowed, at least one of the other, more traditional methods was also used for cross checking.•No significant difference was found.

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Estimated volume of sediment retained behind a weir was calculated by subtracting the volume of two triangular based pyramids based on field measurements.

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•Sediment samples were fractionated for P with the same method, and, similarly, converted into bioavailability ratios.

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•The same statistics order of operations. •Again, alpha was set to .1•χ2 = Test Statistic for the one-way test•S = Test Statistic test statistic for the two sample test

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In this study, weirs are clearly effective at retaining water better than controls, with weirs collectively having an average water depth 8 times greater than controls

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Collectively, weirs retain twice as much sediment as the controls.

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Split among ages, most of this sediment is retained behind one year old weirs.The rapid pace of sediment accumulation observed underscores the critical need for weirs in drainage ditches to intercept the quantity of sediment headed for downstream waters from the agricultural landscape.

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Because volume of sediment depends on dimensions of the ditch and weir, comparisons among ages can’t be made, however, substantial volumes of sediment are quantitatively being withheld behind weirs and preventing impacts downstream

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Despite the differences in water depth, there was no significant difference in pH either between weirs and controls nor among weir ages.

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Nor was there a significant difference in bioavailability ratio between weirs and controls.

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However, spilt among ages, one year old weirs had the lowest bioavailability ratio.

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•These results suggest annual maintenance would be required to reset the system for optimal sediment and P reduction, based on one year old weirs having the greatestsediment depth and the lowest bioavailability ratio. •Additionally, short term biological P removal could bestow additional benefit throughout one growing season, eliminating P reintroduction because of annual maintenance already scheduled based on sediment accumulation rates.•With sediment loss on a scale this considerable, a rapid solution is not only required, but also justifiable under an annual maintenance plan. •Yearly maintenance may be viewed as labor intensive; however, efforts would be restricted to weir locations. Additionally, excavated sediment from the drainage ditch can be transported upland to supplement upland areas experiencing soil loss. This confers further benefit by transporting associated P with previously inundated sediment to a terrestrial system, where through drying and rewetting from rain or irrigation, P becomes available for crop production, decreasing input costs.

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•Agricultural surface drainage ditch weirs can be looked at as sharing characteristics of constructed wetlands, headwater streams, vegetated buffer strips, and floodplains, but should be considered their own distinct system due their accelerated dynamics. •Weirs implemented in agricultural ditches are intended to use hydrologic manipulation to mimic wetland ecosystem services of sediment and P retention, but with some additional benefits:•Drainage ditches are a preexisting, and readily available feature on the agricultural landscape. Consequently weirs installed in drainage ditches do not require land to be taken out of production. They can be constructed out of a range of building materials with various costs. Multiple small weirs can be arranged within a drainage system to allow for a compounding positive water quality effect while allowing a flexible design.Drainage ditches are a logical target for nutrient mitigation because they are primary conduits and already stressed systems.•The application of weirs in agricultural drainage ditches is new, but it would seem that their effects on hydrology and P dynamics occur quickly after installation. •And, although they alter environmental conditions known to release P, by holding a steady hydraulic residence time, they can still be utilized as an effective nutrient management tool. •These results support using weirs to manipulate agricultural drainage ditch hydrology for reduction of P contribution to eutrophication of receiving waters

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