Influences on tidal channel and aquaculture shrimp pond water chemical composition in Southwest Bangladesh

Matthew Dietrich,John C Ayers

doi:10.1186/s12932-021-00074-2

Abstract

Detailed geochemical studies of both major and minor elements in Bangladesh surface waters are sparse, particularly in shrimp aquaculture pond environments. Therefore, water samples from shrimp aquaculture ponds and tidal channels were collected in high precipitation (July) and low precipitation (May) months from 2018–2019 in Southwest Bangladesh and analyzed for complete water chemistry. Selenium (Se) and arsenic (As) were elevated above WHO guidelines in 50% and ~ 87% of samples, respectively, but do not show any recognizable spatial patterns. Shrimp pond and tidal channel water compositions in the dry season (May) are similar, illustrating their connectivity and minimal endogenous effects within shrimp ponds. Tidal channels are less saline in July than shrimp ponds still irrigated by tidal channels, suggesting that either farmers limit irrigation to continue farming saltwater shrimp, or the irrigation flux is low and leads to a lag in aquaculture-tidal channel compositional homogenization. δ18O and δ2H isotopic compositions from samples in May of 2019 reveal tidal channel samples are closer to the local meteoric water line (LMWL) than shrimp pond samples, because of less evaporation. However, evaporation in May shrimp ponds has a minimal effect on water composition, likely because of regular drainage/exchange of pond waters. Dissolved organic carbon (DOC) is positively correlated with both δ18O and δ2H in shrimp ponds, suggesting that as evaporation increases, DOC becomes enriched. Multiple linear regression reveals that As and Se can be moderately predicted (adjusted R2 values between 0.4 and 0.7, p < 0.01) in surface waters of our study with only 3–4 independent predictor variables (e.g., Ni, V and DOC for Se prediction; Cu, V, Ni and P for As prediction). Thus, this general approach should be followed in other regions throughout the world when measurements for certain hazardous trace elements such as Se and As may be lacking in several samples from a dataset.

Highlights

Introduction there has been much research in Bangladesh on groundwater and contaminants such as arsenic (As) (e.g., [4, 7, 17, 38, 47, 48]), less emphasis has been placed on surface water chemistry, especially in Southwest Bangladesh
Studies have looked at trace element concentrations in aquaculture ponds near the coastal region of Bangladesh [23, 32, 62] or in coastal Bangladesh rivers [36, 52] and nonspecified lakes/ponds [1]
Studies far have not thoroughly researched the relationship of tidal channel waters with aquaculture shrimp ponds, which are often irrigated with tidal channel water during the dry season in Southwest Bangladesh instead of groundwater [6] (Fig. 1)

Summary

Introduction

There has been much research in Bangladesh on groundwater and contaminants such as arsenic (As) (e.g., [4, 7, 17, 38, 47, 48]), less emphasis has been placed on surface water chemistry, especially in Southwest Bangladesh. Studies have looked at trace element concentrations in aquaculture ponds near the coastal region of Bangladesh [23, 32, 62] or in coastal Bangladesh rivers [36, 52] and nonspecified lakes/ponds [1] These studies have focused more on risk assessment, general reporting of trace element or major element concentrations, and overall water quality. Studies far have not thoroughly researched the relationship of tidal channel waters with aquaculture shrimp ponds, which are often irrigated with tidal channel water during the dry season in Southwest Bangladesh instead of groundwater [6] (Fig. 1). Recent studies indicate that tidal channel waters often have elevated arsenic, especially in the dry season [6] [25], so there is concern that shrimp ponds and the shrimp grown in them may have high arsenic

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