Seasonal and interannual variations of nitrate and chloride in stream waters related to spatial and temporal patterns of groundwater concentrations in agricultural catchments

Abstract

Abstract:Nitrate concentrations in streamwater of agricultural catchments often exhibit interannual variations, which are supposed to result from land-use changes, as well as seasonal variations mainly explained by the effect of hydrological and biogeochemical cycles. In catchments on impervious bedrock, seasonal variations of nitrate concentrations in streamwater are usually characterized by higher nitrate concentrations in winter than in summer. However, intermediate or inverse cycles with higher concentrations in summer are sometimes observed. An experimental study was carried out to assess the mechanisms that determine the seasonal cycles of streamwater nitrate concentrations in intensive agricultural catchments. Temporal and spatial patterns of groundwater concentrations were investigated in two adjacent catchments located in south-western Brittany (France), characterized by different seasonal variations of streamwater nitrate concentrations. Wells were drilled across the hillslope at depths ranging from 1Ð5 to 20 m. Dynamics of the water table were monitored and the groundwater nitrate and chloride concentrations were measured weekly over 2 years. Results highlighted that groundwater was partitioned into downslope domains, where denitrification induced lower nitrate concentrations than into mid-slope and upslope domains. For one catchment, high subsurface flow with high nitrate concentrations during high water periods and active denitrification during low water periods explained the higher streamwater nitrate concentrations in winter than in summer. For the other catchment, the high contribution of groundwater with high nitrate concentrations smoothed or inverted this trend. Increasing bromide/chloride ratio and nitrate concentrations with depth argued for an effect of past agricultural pressure on this catchment. The relative contribution of flows in time and correlatively the spatial origin of waters, function of the depth and the location on the hillslope, and their chemical characteristics control seasonal cycles of streamwater nitrate concentrations and can influence their interannual trends.

Spatial distribution of soils determines export of nitrogen and dissolved organic carbon from an intensively managed agricultural landscape

“…Nutrients are mobilised by mineralisation processes during the growing season when high rates of microbial activity are observed (Böhlke and Denver, 1995;Böhlke, 2002;Schnabel et al, 1993). NO − 3 leached to shallow groundwater may be steadily released through time periods of a year or more (Martin et al, 2004;Molénat et al, 2002;Ruiz et al, 2002b). As shown by Schiff et al (2002), NO − 3 concentrations can be traced back to groundwater charges and might give a correct basis to explain the almost constant NO − 3 -N concentrations measured at sampling point 1 over the whole sampling period.…”

Solute transport dynamics in small, shallow groundwater-dominated agricultural catchments: insights from a high-frequency, multisolute 10 yr-long monitoring study

“…What is the appropriate timescale to identify the water quality response to a specific human pressure? In the literature focusing on catchment scale responses, three timescales are generally investigated: (i) short-term studies, mostly at minute or hour sampling frequency, analyse the variations of water chemical composition during storm or flood events Williams, 1989), (ii) long-term studies, defined as data series of at least 20 yr of water quality monitoring (Burt and Worrall, 2009), mostly sampled at a weekly frequency, focus on decadal trends Howden et al, 2010;Hrachowitz et al, 2013;Monteith et al, 2000), and (iii) intermediate scale studies focus on seasonal variations (Dawson et al, 2008(Dawson et al, , 2011Martin et al, 2004;Mulholland and Hill, 1997). This intermediate timescale highlights annual variations that repeat themselves from year to year, enabling interannual variability analysis.…”

Timescales of regional circulation of saline fluids in continental crystalline rock aquifers (Armorican Massif, western France)

“…Evapotranspiration in Brittany represents about 50-60 % of the total precipitation, and cannot provide chloride concentrations in recharge water higher than 40 mg L −1 (Ayraud et al, 2008). Agricultural pollution may increase the concentrations to 50 mg L −1 in the most intensively farmed areas (Martin et al, 2004). Thus, these two processes cannot explain the very high concentrations observed.…”

“…If we exclude a 1 km band across the coastline, the meteoric waters in Brittany have a maximum mean chloride concentration of 10-18 mg L −1 with a relatively low variation range (Ayraud et al, 2008;Martin et al, 2004). Evapotranspiration in Brittany represents about 50-60 % of the total precipitation, and cannot provide chloride concentrations in recharge water higher than 40 mg L −1 (Ayraud et al, 2008).…”