Spatio-temporal Distribution of Atmospheric Chloride Deposition over a Small Island

Estimating chloride deposition rates is important for several applications. The conservative nature of chloride makes it a widely used natural tracer for hydrological investigations, such as groundwater recharge estimations and catchment salt balances. Additionally, chloride deposition is used to identify the sources and pathways of air pollutants as well as predict the likelihood of corrosion in infrastructures. The variability of atmospheric chloride (Cl-) deposition is a crucial consideration in hydrological investigations, particularly in coastal areas where it can fluctuate greatly. On small islands, spatial variations of chloride deposition have been found to be particularly strong, which may lead to large estimation errors when assuming point measurements are representative of larger areas. This paper aims to improve knowledge of the factors affecting chloride deposition's spatial and temporal variability in small islands using Norfolk Island (South Pacific) as a case study. A monitoring network consisting of 15 open-site rainwater collectors was installed, from which 275 rainwater samples were collected for the period September 2020-July 2022. Total deposition rates for this period varied from 22.7 to 36.7 mg m-2 d-1 for sites located more than 400 meters from the coast. A higher variation is observed closer to the coast, with values varying from 50.9 to 1022.7 mg m-2 d-1 for sites located between 50 and 200 m from the coast. Statistical analysis shows that distance from the nearest point on the coast and elevation above sea level are significant factors contributing to spatial variability of chloride deposition, whereas period-total rainfall depth and period-average maximum daily wind gust speed significantly affect the temporal variability. This study contributes to knowledge on chloride deposition variability as well as the understanding of factors contributing to this variability in small islands. Additionally, a double exponential model for spatial distribution of chloride deposition in small islands is proposed. The variability of atmospheric chloride (Cl-) deposition is a crucial consideration in hydrological investigations, particularly in coastal areas where it can fluctuate greatly. Our analysis of a 35.7 km2 small island revealed that chloride deposition exhibited significant variation over both space and time. Multiple regression analysis identified four factors - distance from coast, elevation, rainfall, and wind gust - that explained approximately 60% of the variability in chloride deposition.image