Low-Level 222Rn-in-Water Measurement in Arid Aquifers: Method Optimization and a Transferable Monitoring Framework for Sustainable Water Management

Abstract

Reliable surveillance of dissolved 222Rn in arid-region aquifers is challenged by very low natural activity and method-dependent biases, especially humidity sensitivity in electrostatic detectors and air–water partitioning during closed-loop aeration, which can obscure true concentrations needed for defensible drinking-water baselines under preventive frameworks. This study aimed to optimize and field-validate a low-background RAD7 Big-Bottle (RAD H2O) closed-loop protocol tailored for arid conditions and apply it in a regional survey of groundwater used for potable supply in northeastern Saudi Arabia. Groundwater from wells across the region (shallow and deep completions) was collected and analyzed using isotope-resolved alpha spectroscopy (Po-218 and Po-214 windows) with strict chamber humidity control (≤7% RH), background checks, systematic blanks, duplicates, drift control (±10%), and uncertainty propagation. Air-phase chamber counts were mandatorily converted to water-phase activity using the CAPTURE parameterized by measured loop volumes, temperature, salinity, and humidity, and agreement was evaluated using regression diagnostics and Bland–Altman analysis. The optimized method achieved sub-Bq·L−1 performance, with MDL improving from ~0.1645 Bq·L−1 (30 min) to ~0.0233 Bq·L−1 (1500 min) and ~0.0165 Bq·L−1 (3000 min), and LOQ decreasing from ~0.50 to ~0.0707 and ~0.050 Bq·L−1, respectively. Raw air-phase readings systematically overestimated dissolved radon by ~26% (slope ≈ 1.26), a bias removed by the validated air → water conversion. Surveyed 222Rn concentrations were uniformly low (0.03–3.20 Bq·L−1), far below commonly used reference values (e.g., ~11.1 and ~100 Bq·L−1), with no persistent spatial hotspots and broadly overlapping shallow/deep distributions, indicating variability dominated by local lithology and fracture-controlled flow rather than depth. A tiered monitoring scheme is recommended: short screening, routine baselining at ~900–1500 min total counting, and ~3000 min for ultralow verification, providing a transferable template for sustainable baseline programs in arid aquifers.