Assessing the effect of urbanization on regional-scale surface water-groundwater interaction and nitrate transport

Abstract

Identifying regional-scale surface water-groundwater interactions (SGI) is vital for predicting anthropogenic effects on surface water bodies and underlying aquifers. However, large-scale water and nutrient flux studies rely on surface water or groundwater-focused models. This study aims to model the effect of urbanization, which is usually accompanied by high groundwater abstraction and surface water pollution, particularly in the developing world, on a regional-scale SGI and nitrate loading. In the study area, the urban expansion increased by over 3% in the last decade. The integrated SWAT-MODFLOW model, Soil and Water Assessment Tool (SWAT) and Modular Finite-Difference Groundwater Flow (MODFLOW) coupling code, was used to assess SGI. By coupling SWAT-MODFLOW with Reactive Transport in 3-Dimensions, the nutrient loading to the river from point and non-point sources was also modeled. Basin average annual results show that groundwater discharge declined with increasing groundwater abstraction and increased with Land use/Land cover (LULC) changes. Groundwater recharge decreased significantly in the Belge season (February to May), and the river seepage and groundwater discharge decreased correspondingly. High spatiotemporal changes in SGI and nitrate loading were found under the combined LULC and groundwater abstraction scenarios. The water yield decreased by 15%. In a large part of the region, the nitrate loading increased by 17-250%. Seasonally controlled groundwater abstraction and water quality monitoring are essential in this region.

Figure 1

Description of the study area, including Major River networks, gauging stations, and reservoirs processed using QGIS3.10.4 (https://www.qgis.org/en/site/).

Figure 2

SWAT model inputs mapped using QGIS3.10.4 (https://www.qgis.org/en/site/): (a) Digital Elevation Model (DEM 30 m, USGS; https://earthexplorer.usgs.gov/), (b) Soil Classes, (c) Slope classes, and (d) Land Use/Land cover (LULC) for the year 2000.

Figure 3

Geology and hydrogeology of Akaki River Basin, obtained from the Ministry of Water Resources, Irrigation, and Electricity (EMWRIE), the principal boundary conditions considered in the groundwater flow modeling, and location of contaminant point sources, processed using QGIS3.10.4 (https://www.qgis.org/en/site/).

Figure 4

Conceptual diagram showing surface and subsurface processes and modeling framework, adopted and modified from previous studies^,,, with copyright permission.

Figure 5

Calibration and validation of river flow at Little Akaki (a) and Big Akaki (b) gauging stations and the average precipitation in the study area.

Figure 6

Map showing the calibrated starting groundwater head (a), the spatial distribution of groundwater discharge to the river (b), the initial nitrate concentration (c) in the Akaki Aquifers prepared using the QSWATMOD2 plugin (https://swat.tamu.edu/software/swat-modflow/), and scatter plot of measured and simulated groundwater head (d).

Figure 7

Comparison of SWAT-MODFLOW simulated groundwater discharge and baseflow in Little Akaki and Big Aaki Rivers.

Figure 8

Average monthly groundwater recharge and water yield (a) and seasonal groundwater recharge fluctuation from 1990 to 2013 (b–d) in the Akaki River Basin.

Figure 9

Seasonal surface water discharge to the aquifer (swgw) and groundwater discharge to the river (gwq) differences under the simulation scenarios from the baseline scenario (scenarios-bassline).

Figure 10

Groundwater level difference from the baseline scenario from 1990 to 2013: positive values show that the groundwater level decreased compared to the baseline simulation.

Figure 11

Spatial distribution of average groundwater recharge under baseline (a), scenario 2 (b), and scenario 4 (c) in the study area from SWAT-MODFLOW output prepared using QSWATMOD2 (https://swat.tamu.edu/software/swat-modflow/).

Figure 12

Temporal distribution of average nitrate (No3) seepage to aquifer: (a) comparison of average seasonal No3 seepage in each simulation scenario, (b–d) Seasonal No3 seepage from 1990 to 2013 under all simulation scenarios.

Figure 13

Spatial distribution of nitrate (NO3) seepage (Kg/day) from the river to the aquifer: (a) baseline, (b) scenario 1, (c) scenario 2, (d) scenario 3, (e) Scenario 4, and (f) Scenario 5.