Predicted uranium and radon concentrations in New Hampshire (USA) groundwater—Using Multi Order Hydrologic Position as predictors

Abstract

Two radioactive elements, uranium (U) and radon (Rn), which are of potential concern in New Hampshire (NH) groundwater, are investigated. Exceedance probability maps are tools to highlight locations where the concentrations of undesirable substances in the groundwater may be elevated. Two forms of statistical analysis are used to create exceedance probability maps for U and Rn in NH groundwater. The first, Boosted Regression Tree (BRT), was selected for estimating U exceedance values. It computes exceedance values directly using the Bernoulli distribution function. The second method of statistical analysis used for Rn to determine exceedance probabilities is ordinary least squares (OLS) regression. In the process of determining exceedance probabilities for U and Rn, the utility of a new dataset is investigated. That new predictor dataset is the Multi‐Order Hydrologic Position (MOHP) dataset. MOHP raster datasets have been produced nationally for the conterminous United States at a 30‐m resolution. The concept behind MOHP is that, for any given point on the earth's surface, there is the potential for a longer groundwater flow path as one goes deeper beneath the land surface. MOHP predictors were tested in both models. Three MOHP predictors were found useful in the BRT model and two in the OLS model. MOHP data were found useful as predictors along with other site characteristics in predicting U and Rn exceedance probabilities in New Hampshire groundwater.

Estimating Lithium Concentrations in Groundwater Used as Drinking Water for the Conterminous United States

“…The lateral position (LP) of a well is a normalized dimensionless value ranging from 0 to 10,000 that represents the location of a well relative to its distance between a stream of n th order and the drainage divide. , The lower the value, the closer the well is to a stream where the LP is equal to zero. Lateral positions have been calculated for the CONUS for first- through ninth-order streams ( n = 1 through 9), , we use 5 of the 9 LP variables (LP 1, 3, 5, 7, 9) in our model to avoid over-reliance on these variables. The SHAP plot for the LP for ninth-order streams (LP_9), which has the highest relative importance of the LP variables, is shown in Figure c and indicates a nonlinear relationship where Shapley values are greatest at low (<1000) and high (>9000) LP values and dip in between.…”

“…The final model contains 20 predictor variables ( Figure 3 and Table SI_3 ). They include climatic variables such as average annual precipitation, 40 geochemical variables representing the soil chemistry, 41 and hydrologic variables such as the lateral position of a well with respect to streams and their hydrologic divides 42 and outputs from a national groundwater model. 43 The variable importance within the model was determined using the function built into the XGBoost package that calculates the fractional contribution of each feature based on the total gain from the splits that use that feature.…”

“…20 The lateral position (LP) of a well is a normalized dimensionless value ranging from 0 to 10,000 that represents the location of a well relative to its distance between a stream of nth order and the drainage divide. 42,46 The lower the value, the closer the well is to a stream where the LP is equal to zero. Lateral positions have been calculated for the CONUS for firstthrough ninth-order streams (n = 1 through 9), 42,46 we use 5 of the 9 LP variables (LP 1, 3, 5, 7, 9) in our model to avoid over-reliance on these variables.…”

Prioritizing water availability study settings to address geogenic contaminants and related societal factors

“…3 and 4 , S2 ). In areas where samples have been collected, high proportions of samples exceed thresholds for As, Sr, and Se (Table 1 ) (Ayotte et al, 2003 ; Moore et al, 2022 ). Ravalli et al ( 2022 ) show the central southwestern USA as an area of high concentrations of As, Ba, Cr, Se, and U in community water systems.…”