A case study is presented for the application of statistical and geostatistical methods to the problem of estimating groundwater quality variables. This methodology has been applied to the investigation of the detrital aquifer of the Bajo Andarax (Almeria, Spain). The use of principal components analysis is proposed, as a first step, for identifying relevant types of groundwater and the processes that bring about a change in their quality. As a result of this application, three factors were obtained, which were used as three new variables (VI: sulphate influence; V2: thermal influence; and V3: marine influence). Analysis of their spatial distribution was performed through the calculation of experimental and theoretical variograms, which served as input for geostatistical modelling using ordinary block kriging. This analysis has allowed a probabilistic representation of the data to be obtained by mapping the three variables throughout the aquifer for each sampling point. In this way, one can evaluate the spatial and temporal variation of the principal physico-chemical processes associated with the three variables VI, V2 and V3 implicated in the groundwater quality of the detrital aquifer.
This study examines the relationship between dissolved inorganic nitrogen (DIN: NH 4 + + NO 2 − +NO 3 − ) and dissolved organic nitrogen (DON) in river water and groundwater in an agriculturallydominated catchment in south-east Spain, and estimates the contribution of DON to the total dissolved nitrogen (TDN) concentrations. The studied aquiferdependent river system consists of Quaternary alluvial sediments deposited by the Guadalquivir River and its tributaries, with both river water and groundwater in the catchment being aerobic. DON is the predominant form of nitrogen in river water (72−97% of the TDN), whereas its proportion to TDN varies considerably in groundwater (<1−99%). A seasonal pattern in the concentration of dissolved nitrate in river water was observed, whereas DON concentrations showed no significant change during the study period. The export of DON from the Guadalquivir River is approximately 2 kg N ha −1 year −1 and is an order of magnitude higher than the export of DON from pristine catchments. Dissolved nitrate concentrations in groundwater were slightly higher in winter and DON concentrations were significantly lower (p<0.05) in summer. It is found that agricultural soils constitute the main storage of organic nitrogen in the catchment with a steady leaching of high DON concentrations into the alluvial aquifer system (mean value 19.1 mg N L −1 ), and so indicating that DON should not be overlooked in the nitrogen budgets of agriculturallydominated catchments.
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