The impact of seawater diversion into an Estuarine System, St. Lucia, South Africa

Abstract

Coastal aquifers such as the unconfined aquifer in St. Lucia, KwaZulu-Natal, South Africa, are susceptible to seawater intrusion which may be exacerbated by the impacts of climate change and increasing coastal populations. Residents and businesses are largely dependent on groundwater for daily use due to poor municipal water supply. To determine whether fresh groundwater was being contaminated through mixing with seawater, groundwater samples were collected from boreholes in the St. Lucia town and Khula and Duku villages further inland. Physicochemical, hydrochemical and stable water isotope analyses (δ2H and δ18O) were conducted on a total of 52 samples collected from the sea, estuary, river and groundwater in January and February 2021. The stable isotope analysis was conducted at the WITS Hydrogeology Laboratory while the hydrochemical analyses were outsourced to an external SANAS accredited laboratory in Pretoria. The results showed that for most samples, low electrical conductivity (EC) values combined with low Cl/Br ratios and isotopic mixing proportions closer to that of fresh groundwater, indicate that seawater intrusion did not occur to a great extent within the uppermost water-bearing hydrostratigraphic unit(HSU). This HSU iscomposed of sands, clays,and silts, and remains the primary unit from which groundwater is abstracted. The position of the Cretaceous aquicludebelow the water-bearing HSU, seaward flow of groundwater and estuary-produced freshwater lens serve as important buffers for the inland advancement of seawater as conceptualised in a seawater interface model. Two groups of groundwater were determined using a Piper plot and confirmed with an agglomerative hierarchical clustering analysis (HCA) which indicates that different processes are at play to produce each group. Group 1 waters are of Na-Cl type which have likely undergone recycling through evaporation, infiltration,and precipitation, while group 2 waters are of Ca-HCO3+CO3 type with the calcarenites of the Uloa Fm and calcareously cemented dune sands of the Kosi Bay Fmbeing possible source aquifers.Some samples exhibit hydrochemical or isotopic signatures close to that of seawater indicating minor mixing due to high rates of groundwater abstraction and concentration of Cl, Mg and Na by evaporation, septic tank waste, animal waste and agricultural return flows. Groundwater within the study area is considered largely uncontaminated by seawater intrusion, however, continuous monitoring of water levels and physicochemical variables is advised to prevent seawater intrusion in the future and maintain or improve the current groundwater quality.