Tidally Forced Saltwater Intrusions might Impact the Quality of Drinking Water, the Valdivia River (40° S), Chile Estuary Case.

Yongshan Wan,Wolfgang Schneider,Francisco Olguin,José Garcés-Vargas,Andrea Piñones,Daniel Brieva,Andre Pinochet

doi:10.3390/w12092387

Abstract

The Valdivia River estuary (VRE) located in south-central Chile is known as one of the largest estuarine ecosystems on the Pacific coast. This research aims to determine the intra-tidal and sub-tidal variability of saline intrusions into the VRE between November 2017 and March 2019 derived from salinity sensors located along the VRE. Complementary hydrographic measurements were conducted during flood and ebb conditions of the spring and neap tides for each of the four seasons of the year along the central axis of the VRE. The results of the salinity time series showed that saline intrusions (values greater than 0.5 Practical Salinity Units) occurred ~20 km from the estuary mouth, when the total flow of the Cruces and Calle-Calle rivers (main tributaries of the estuary) was low, around 280–300 m3 s−1. During the same period, the best co-variability was observed between the saline intrusions and the mixed-semidiurnal tide and the fortnightly and monthly periods of the tide. Regression analyses indicated that salinity intrusion length (L) is best correlated to discharge (D) with a fractional power model L α D−1/2.64 (R2 = 0.88). The decreasing discharge trend, found between 2008–2019, implies that saline water intrusions would negatively impact the Valdivia’s main drinking water intake during the low rainfall season under future climate conditions.

Highlights

In positive estuaries, the seawater propagates over the bottom of the river mouth, due to the difference in density between the upper fresh and lower haline water and moves upstream against the river flow
Our analysis showed that the saltwater intrusions occurred ~20 km from the Valdivia estuary mouth, when the discharges were below the threshold of 280–300 m3 s−1
Study we investigated the extent of saltwater intrusions into the Valdivia River estuary (VRE) on intra-tidal (12–25 h) and at the bottom of the river maximum salinities occurred about an hour later during flood tides with sub–tidal (>1 day) time scales in relation to total river discharge throughout the four seasons of the slightly higher salinity, up to more than 10 Practical Salinity Units (PSU) at Valdivia station

Summary

Introduction

The seawater propagates over the bottom of the river mouth, due to the difference in density between the upper fresh and lower haline water and moves upstream against the river flow. During times of minimum river discharge, the length of seawater intrusion can negatively affect the quality of water used for domestic, agricultural, industrial, and other uses. Under future scenarios of sea level rise, enhanced salinity intrusions in the estuarine system along the coastal zone of Bangladesh is projected to impact the farming industry by substantially reducing yields [2]. Increase in salinity of estuarine systems serves as a key driver of saltwater intrusion into unconfined coastal aquifers, inserting further stress on limited freshwater resources in coastal zones. This issue can become especially pronounced under future climate conditions due to sea level rise, reduced freshwater inflow, or increased demand of freshwater resources. Delineating the salinity structure in estuarine systems will help in setting accurate boundary conditions for both a semi-analytical solution of the Henry saltwater intrusion problem [5]

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