Abstract
In agricultural catchments where groundwater (GW) base flow discharge contributes substantially towards stream flow, the information linking GW inflow/outflow with contaminant import/export is scarce. However, this information is essential to address aquatic ecosystem health hazard/risk associated with nitrate export and subsequent loading in sensitive surface water bodies (SWB). The objectives of this study were to assess the temporal dynamics of (i) rain water inflow/outflow behaviour in three agricultural catchments in the humid tropics of far-northeast Queensland of Australia, (ii) solute import via inflow and subsequent export in outflow, and (iii) the association between GW inflow/outflow and solute import/export. Approximately 71% of the average seasonal rainfall percolated (inflow) into the porous basaltic regolith of the Johnstone River Catchment (JRC) compared with 44% into the alluvial regolith in the Mulgrave River Catchment (MRC) and 29% into the metamorphic regolith in the Tully River Catchment (TRC), respectively. The outflows from the basaltic, alluvial, and metamorphic regoliths were 56%, 36%, and 55% of the inflows, respectively. The cumulative nitrate import per season was 25 k/ha in the JRC compared with 11 kg/ha in MRC and 34 kg/ha in TRC. The corresponding exports were 24 kg/ha, 8 kg/ha 26 kg/ha in JRC, MRC, and TRC, respectively. The total dissolved solute (TDS) exports were 82%, 77%, 75%, of the corresponding imports in JRC, MRC, and TRC, respectively. Simple correlations indicated that nitrate export was positively correlated with the outflow in each one of the regolith and similar trends were observed between inflow and import. The import/export mass balance for nitrate shows that 73% to 96% of the imports were exported during the same rainy season, suggesting the potential for nitrate associated ecosystem health hazard/risk in sensitive SWB receiving the outflows.
Highlights
In water balance models, solute import/export at different scales ranging from point measurement to catchment scale in space over-time into and out from groundwater (GW) is usually linked to inflows into and outflows from aquifers [1]-[8]
Reliable information in GW inflow/outflow is important in situations where GW base flow discharges contribute substantially towards the total annual flow in streams, carrying nutrient contaminants such as N and P, and pesticides discharge the flows into sensitive surface water bodies (SWB) [4]-[11]
The limitations are partially attributed to the complexities involved in characterizing and quantifying GW inflow/outflow from point measurement extrapolated to larger scale and over-time [3] [6] [7] [8] [9]
Summary
Solute import/export at different scales ranging from point measurement to catchment scale in space over-time into and out from groundwater (GW) is usually linked to inflows into and outflows from aquifers [1]-[8]. Of particular importance with regard to point measurement, outflow and solute export are GW-head [3], temperature-time series [7], and recharge an discharge [5]. These workers have indicated that model outputs may be less reliable, in situations where the information in the aforementioned major variables listed in [1]-[8] is scarce. Reliable information in GW inflow/outflow is important in situations where GW base flow discharges contribute substantially towards the total annual flow in streams, carrying nutrient contaminants such as N and P, and pesticides discharge the flows into sensitive surface water bodies (SWB) [4]-[11]. Coupling GW inflow/outflow with solute import/export spatiotemporally may help to at least partially resolve the issues mentioned above, to my knowledge, such coupling information is scarce
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