Predicted nitrate and arsenic concentrations in basin-fill aquifers of the Southwest Principal Aquifers study area

This product "Predicted nitrate and arsenic concentrations in basin-fill aquifers of the 
Southwest Principal Aquifers study area" is a 1:250,000-scale vector dataset and was 
developed as part of a regional Southwest Principal Aquifers (SWPA) study. The study 
examined the vulnerability of basin-fill aquifers in the southwestern United States to 
nitrate contamination and arsenic enrichment. Statistical models were developed by 
using the random forest classifier algorithm to predict concentrations of nitrate and 
arsenic across a model grid that represents local- and basin-scale measures of 
source, aquifer susceptibility, and geochemical conditions. Separate classifiers  
were  developed for nitrate and arsenic because each constituent was expected to 
be affected by a different set of factors, and each factor could have a different 
magnitude or directional influence (increase/decrease) on concentration. For each 
constituent, two different classifiers were developed; a prediction classifier and a 
confirmatory classifier. The prediction classifiers were developed specifically to 
predict nitrate and arsenic concentrations in basin-fill aquifers across the SWPA 
study area and were based on explanatory variables representing source and 
susceptibility conditions. These explanatory variables were available throughout 
the entire SWPA study area and, therefore, did not pose a limitation for using the 
classifiers to predict concentrations.
		
The confirmatory classifiers were developed to supplement the prediction classifiers 
in the evaluation of the conceptual model. The name, "confirmatory," reflects the 
classifier's purpose for evaluation of a-priori hypotheses and contrasts other general 
types of statistical models, such as those used for prediction or exploratory purposes. 
The confirmatory classifiers included the explanatory variables used in the prediction 
classifiers, as well as additional variables representing geochemical conditions and 
basin groundwater budget components. The inclusion of the geochemical and basin 
groundwater budget variables in the confirmatory classifiers allowed for further 
evaluation of the conceptual models, which was not possible with the prediction 
classifiers alone. The geochemical data, however, were only available at specific 
well locations, and consistent water-budget data were not available for every basin 
in the study area. The limited availability of the data for these variables constrained 
the confirmatory classifiers to observations from 16 case-study basins and precluded 
use of the confirmatory classifier for predicting concentrations across the SWPA 
study area. To contrast the scope of the two classifiers, the confirmatory classifiers 
were developed by using all available explanatory variables but with observations 
restricted to the 16 case-study basins, whereas the prediction classifiers were 
unrestricted with respect to spatial extent because these were developed by 
using a subset of the explanatory variables that were available throughout the 
study area.

Tags: arizona, arsenic-concentration, basin-fill-aquifer, california, colorado, environment, geoscientificinformation, groundwater, groundwater-contamination, groundwater-susceptibility, inlandwaters, national-water-quality-assessment-program, nawqa, nevada, new-mexico, nitrate-concentration, southwest-united-states, usgs-ae8e9136-9b70-4612-a184-3aebde4c922e, utah, utilitiescommunication, water-quality
Organization: Department of the Interior