François Birgand

Uncertainties in assessing annual nitrate loads and concentration indicators: Part 1. Impact of sampling frequency and load estimation algorithms.

The objectives of this study are to evaluate the uncertainty in annual nitrate loads and concentrations (such as annual average and median concentrations) as induced by infrequent sampling and by the algorithms used to compute fluxes. A total of 50 watershed-years of hourly to daily flow and concentration data gathered from nine watersheds (5 to 252 km²) in Brittany, France, were analyzed. Original (high frequency) nitrate concentration and flow data were numerically sampled to simulate common sampling frequencies. Annual fluxes and concentration indicators calculated from the simulated samples were compared to the reference values calculated from the high-frequency data. The uncertainties contributed by several algorithms used to calculate annual fluxes were also quantified. In all cases, uncertainty increased as sampling intervals increased. Results showed that all the tested algorithms that do not use continuous flow data to compute nitrate fluxes introduced considerable uncertainty. The flow-weighted average concentration ratio method was found to perform best across the 50 annual datasets. Analysis of the bias values suggests that the 90th and 95th percentiles and the maximum concentration values tend to be systematically underestimated in the long term, but the load estimates (using the chosen algorithm) and the average and median concentrations were relatively unbiased. Great variability in the precision of the load estimation algorithms was observed, both between watersheds of different sizes and between years for a particular watershed. This has prevented definitive uncertainty predictions for nitrate loads and concentrations in this preliminary work, but suggests that hydrologic factors, such as the watershed hydrological reactivity, could be a key factor in predicting uncertainty levels.

AgriBMPWater: systems approach to environmentally acceptable farming

To help local regulators mitigate non-point source agricultural pollution and implement environment-friendly agricultural practices, a comparison between different existing or simulated best management practices (BMPs) has been carried out within a pluridisciplinary project called AgriBMPWater (FP5 founded). The project has been imagined and built in a pluridisciplinary approach and framework. The approach developed corresponds to a cost/effectiveness assessment of several BMPs in several European watersheds, also including the study of their acceptability by farmers. Thanks to the integrated assessment of existing and potential BMPs, a selection grid contributes to provide assistance to regulators on how to conduct environmental, economic and sociological analyses for helping decision makers. Water quality problems encountered and dealt with in this project include nitrate, phosphorus, sediment, pesticide loads and acid water concerns. Thus, the developed framework allows for a large range of hydrological and economic models, depending on the environmental problem detected in each watershed.

Estimating Nitrogen, Phosphorus, and Carbon Fluxes in Forested and Mixed-Use Watersheds of the Lower Coastal Plain of North Carolina: Uncertainties Associated with Infrequent Sampling

Assessing the impact of a land use change or the water quality improvement provided by a treatment system almost always involves computation of the difference in nutrient loads before and after implementation, or upstream and downstream of the system studied. Reporting meaningful values on mass balance or differences in nutrient loads implies that the uncertainty in the computed loads is several times smaller than the difference itself. This may imply very small uncertainties for the nutrient load measurements. The level of uncertainty induced by infrequent sampling on annual loads was investigated for a suite of nutrients in runoff from a forested watershed and a mixed land use watershed in the lower coastal plain of North Carolina. Reference data were used to simulate discrete sampling and to calculate new annual load estimators, which were then compared to the reference loads to calculate the level of uncertainty. Uncertainties depended on the watershed and the nutrients and other constituents, but their level was generally found to be high, around ±20% and ±40% or more for weekly and monthly sampling for most nutrients. This was generally attributed to the short periods of active flow in these watersheds and the flashiness of flow associated with subsurface drainage. The results suggest that to obtain uncertainties of ±2% or ±5% for nitrogen forms, 100 or more than 200 samples over six months of the year might be necessary in the forested and mixed-use watersheds of the lower coastal plain.

Analysis of Consecutive Events for Nutrient and Sediment Treatment in Field-Monitored Bioretention Cells

Previous research demonstrated that nutrient treatment in conventionally drained bioretention cells is dependent upon temperature and varying wetting and drying regimes in the media. This study examines the influence that previous events have on outflow concentrations by analyzing flow-weighted composite samples from four to six consecutive events during three different seasons for two sets of field-monitored bioretention cells in Nashville, NC. The bioretention cells had different media depths (0.6-m versus 0.9-m). As a means to analyze performance from consecutive events, the evolution of cumulative pollutant loads was presented by plotting cumulative load versus cumulative volume. This method of presenting water quality data allows for the direct analysis of event mean concentrations, load reduction, and volume reduction with one graph, as well as describing the seasonal impacts and impacts from consecutive events. Runoff and outflow concentrations were also correlated to media temperature and rainfall characteristics. The overall results of this study showed that conventionally drained bioretention cells mainly convert organic nitrogen, the predominant source of nitrogen in runoff, into nitrate in the aerobic environment present in the media. Nitrate is then exported from the media during subsequent events. The greatest export occurred during the warmer months because higher media temperatures increased microbial activity. Pollen and leaf litter were identified as organic nitrogen and total phosphorus sources because of elevated runoff concentrations that occurred in the spring and autumn. Based on these results, future bioretention studies should strongly consider monitoring consecutive events and this method of data analysis, as they reveal internal processes and allow researchers to draw conclusions that independent event monitoring could not.

Addressing the Fouling of In Situ Ultraviolet-Visual Spectrometers Used to Continuously Monitor Water Quality in Brackish Tidal Marsh Waters

The introduction of portable in situ ultraviolet-visual spectrometers has made possible the collection of water quality parameters at a high frequency in dynamic systems such as tidal marshes. The usefulness of this technology is inhibited by fouling of the instruments optics. In this study, a spectrometer fitted with manufacturer-recommended compressed air optical cleaning was installed in a brackish marsh to determine if fouling interfered with measurements between bi-weekly servicing. During a 2-wk period, the absorbance measured in air at 220 nm increased from 9 to 549 m, indicating major fouling. An antifouling system was developed that reduced the time of exposure of the optics to stream water and used a pressurized fresh water cleaning. After implementation of the system, the absorbance in air increased to at most 63 m after 2 wk of data collection. The dramatic reduction in fouling will allow quality long-term data to be collected using this technology.

Quantifying the fate of agricultural nitrogen in an unconfined aquifer: Stream-based observations at three measurement scales

We compared three stream-based sampling methods to study the fate of nitrate in groundwater in a coastal plain watershed: point measurements beneath the streambed, seepage blankets (novel seepage-meter design), and reach mass-balance. The methods gave similar mean groundwater seepage rates into the stream (0.3–0.6 m/d) during two 3–4 day field campaigns despite an order of magnitude difference in stream discharge between the campaigns. At low flow, estimates of flow-weighted mean nitrate concentrations in groundwater discharge ([ NO3−]FWM) and nitrate flux from groundwater to the stream decreased with increasing degree of channel influence and measurement scale, i.e., [ NO3−]FWM was 654, 561, and 451 µM for point, blanket, and reach mass-balance sampling, respectively. At high flow the trend was reversed, likely because reach mass-balance captured inputs from shallow transient high-nitrate flow paths while point and blanket measurements did not. Point sampling may be better suited to estimating aquifer discharge of nitrate, while reach mass-balance reflects full nitrate inputs into the channel (which at high flow may be more than aquifer discharge due to transient flow paths, and at low flow may be less than aquifer discharge due to channel-based nitrate removal). Modeling dissolved N2 from streambed samples suggested (1) about half of groundwater nitrate was denitrified prior to discharge from the aquifer, and (2) both extent of denitrification and initial nitrate concentration in groundwater (700–1300 µM) were related to land use, suggesting these forms of streambed sampling for groundwater can reveal watershed spatial relations relevant to nitrate contamination and fate in the aquifer.

High-frequency dissolved organic carbon and nitrate measurements reveal differences in storm hysteresis and loading in relation to land cover and seasonality

Storm events dominate riverine loads of dissolved organic carbon (DOC) and nitrate, and are expected to increase in frequency and intensity in many regions due to climate change. We deployed three high-frequency (15-minute) in-situ absorbance spectrophotometers to monitor DOC and nitrate concentration for 126 storms in three watersheds with agricultural, urban, and forested land use/land cover. We examined intrastorm hysteresis and the influences of seasonality, antecedent conditions, storm size, and dominant land use/land cover on storm DOC and nitrate loads. DOC hysteresis was generally anti-clockwise at all sites, indicating distal and plentiful sources for all three streams despite varied DOC character and sources. Nitrate hysteresis was generally clockwise for urban and forested sites, but anti-clockwise for the agricultural site, indicating an exhaustible, proximal source of nitrate in the urban and forested sites, and more distal and plentiful sources of nitrate in the agricultural site. The agricultural site had significantly higher storm nitrate yield per water yield and higher storm DOC yield per water yield than the urban or forested sites. Seasonal effects were important for storm nitrate yield in all three watersheds and farm management practices likely caused complex interactions with seasonality at the agricultural site. Hysteresis indices did not improve predictions of storm nitrate yields at any site. We discuss key lessons from using high-frequency in-situ optical sensors.

Uncertainties in Assessing Annual Nitrate Loads and Concentration Indicators: Part 2. Deriving Sampling Frequency Charts in Brittany, France

In water quality monitoring programs, standard sampling frequency schemes tend to be applied throughout entire regions or states. Ideally, the common standard among monitoring stations ought not to be the sampling frequency but instead the level of uncertainty of the estimated water quality indicators. Until now, there was no obvious way of doing this. This article proposes, for the first time, guidelines to select appropriate sampling frequencies to harmonize the level of uncertainty in the case of yearly nitrate indicators for the regional river water quality monitoring network in Brittany, France. A database of 50 watershed-year datasets (nine watersheds of 4 to 252 km2 in size) was used for which high temporal resolution data (hourly and daily) were available for flow and nitrate concentrations. For each dataset, the uncertainty levels were calculated by numerically simulating sampling intervals varying from 2 to 60 days. The precision limits of the uncertainties were successfully correlated to a hydrological reactivity index. The correlations were used to derive sampling frequency charts. These charts can be used by watershed managers to optimize the sampling frequency scheme for any watershed for a desired uncertainty level, provided that the dimensionless local hydrological reactivity can be calculated from previous records of continuous flow rates. The sampling frequency charts also suggest that, depending on the hydrological reactivity, expected uncertainties generated by monthly sampling range between ±6% and ±14% for the annual load and between -5% and +2.5% to +7.2% for the annual concentration average.

Seasonal variation in the quality of dissolved and particulate organic matter exchanged between a salt marsh and its adjacent estuary

Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and POM) exchanging between a tidal creek in a created salt marsh and its adjacent estuary in eastern North Carolina, USA. Samples from the creek were collected hourly over four tidal cycles in May, July, August, and October 2011. Absorbance and fluorescence of chromophoric DOM (CDOM) and of base-extracted POM (BEPOM) served as the tracers for organic matter quality while dissolved organic carbon (DOC) and base-extracted particulate organic carbon (BEPOC) were used to compute fluxes. Fluorescence was modeled using parallel factor analysis (PARAFAC) and principle components analysis (PCA) of the PARAFAC results. Of nine PARAFAC components (C) modeled, C3 represented recalcitrant DOM and C4 represented fresher soil-derived source DOM. Component 1 represented detrital POM, and C6 represented planktonic POM. Based on mass balance, recalcitrant DOC export was 86 g C m−2 yr−1 and labile DOC export was 49 g C m−2 yr−1; no planktonic DOC was exported. The marsh also exported 41 g C m−2 yr−1 of detrital terrestrial POC, which likely originated from lands adjacent to the North River estuary. Planktonic POC export from the marsh was 6 g C m−2 yr−1. Assuming the exported organic matter was oxidized to CO2 and scaled up to global salt marsh area, respiration of salt marsh DOC and POC transported to estuaries could amount to a global CO2 flux of 11 Tg C yr−1, roughly 4% of the recently estimated CO2 release for marshes and estuaries globally.

Effects of Site Preparation for Pine Forest/Switchgrass Intercropping on Water Quality

A study was initiated to investigate the sustainability effects of intercropping switchgrass ( L.) in a loblolly pine ( L.) plantation. This forest-based biofuel system could possibly provide biomass from the perennial energy grass while maintaining the economics and environmental benefits of a forest managed for sawtimber. Operations necessary for successful switchgrass establishment and growth, such as site preparation, planting, fertilizing, mowing and baling, may affect hydrology and nutrient runoff. The objectives of this study were (i) to characterize the temporal effects of management on nutrient concentrations and loadings and (ii) to use pretreatment data to predict those treatment effects. The study watersheds (∼25 ha each) in the North Carolina Atlantic Coastal Plain were a pine/switchgrass intercropped site (D1), a midrotation thinned pine site with natural understory (D2), and a switchgrass-only site (D3). Rainfall, drainage, water table elevation, nitrogen (total Kjedahl N, NH-N, and NO-N), and phosphate were monitored for the 2007-2008 pretreatment and the 2009-2012 treatment periods. From 2010 to 2011 in site D1, the average NO-N concentration effects decreased from 0.18 to -0.09 mg L, and loads effects decreased from 0.86 to 0.49 kg ha. During the same period in site D3, the average NO-N concentration effects increased from 0.03 to 0.09 mg L, and loads effects increased from -0.26 to 1.24 kg ha. This study shows the importance of considering water quality effects associated with intensive management operations required for switchgrass establishment or other novel forest-based biofuel systems.