Department of Water and Sanitation Zacharia Maswuma Groundwater Resources Assessment of the Nyl River Flood Plain, South Africa, and its relation to the Hydrological Regime of the System during Drought, and proposed Management Options Department of Water and Sanitation South Africa

Introduction This works is desktop study motivated by the following:- The El Nino effect which develops in 2013/14, most parts of the South Africa was declared as disaster stricken area through Government Gazzette; and Also, there has been growing activism by “Environmental Concern Groups” about the decline in groundwater levels in the wetlands affecting biodiversity.

AIMS AND OBJECTIVES Aim The aim of the study is to determine whether current decline recorded groundwater levels monitored in the study area is indicative of the groundwater drought cycle and/or poor management of the aquifer. Objectives Identify the land use activities; Compare relationship between cumulative rainfall and groundwater level; Describe aquifer systems and applicable aquifer characteristics; Develop a theoretical surface-groundwater interaction conceptual model to describe the resource flow regimes in the catchments underlain by aquifers; Recommend measures to manage groundwater resources. Note: The presentation with only address objective 2, correlation between cumulative rainfall (depth) and groundwater level (depth) changes.

Study Area The location and setting of the Nyl and upper Mogalakwena valley and floodplain wetland:- (a) the geology of the Nyl/Mogalakwena valley; and (b) the main tributaries of the Nyl/Mogalakwena River south of the Sterkrivier confluence, highlighting the main area of floodplain wetland. Source: T.S. McCarthy et al, 2011

Physiography Cross section of the flood plain showing some physiographic features; Waterberg foothills rise up to 1 600 meters above sea level; The gradient generates the runoff into the floodplain during heavy rains Source: T.S. McCarthy et al, 2011

Hydrogeological Characteristics Source: T.S. McCarthy et al, 2011

Location of groundwater monitoring stations, SAWS Rainfall station and DWS Gauging Station-stream flow

METHODOLOGY The groundwater level data sets for 11 monitoring station within study area was obtained from DWS NGA & HYDSTRA databases; The rainfall data for one station was obtained from SAWS Station located in Modimolle; The average water level depths and average rainfall recorded for 20 years were analysed for their long-term trend , and were interpreted graphically to understand the dynamics of the groundwater level and rainfall; NB: Only three hydrographs will be presented for station located at upstream and downstream of the floodplain and within the wetland

DISCUSSION AND RESULTS Annual cumulative rainfall and groundwater level at Station A6N0550 ( located in the wetland ) A recharge event occurred between December 2009 and April 2010 followed by sharp declined until November 2013; The groundwater levels in this borehole have a slightly delayed response to rainfall, with recharge occurring approximately three months after high rainfall events; Both boreholes A6N0544, A6N0545 and A6N0553 show the same pattern –shallow boreholes within alluvial formation

Annual cumulative rainfall and groundwater level at Station A6N0083 (Upstream of floodplain) The water level time series trend indicates an increase in water levels until July 2015, followed by a steady declining water level thereafter; A recharge event took place between June 2009 and February 2014, after which decline in water levels is observed and are expected to continue the declining trend; The groundwater levels in this borehole had a slightly delayed response to rainfall, with recharge occurring approximately three months after high rainfall events.

Annual cumulative rainfall and groundwater level at Station Annual cumulative rainfall and groundwater level at Station A6N0059 (downstream of the flood plain) No data from January 1998 until October 2001; A recharge event took place between June 2005 and April 2014, after which a steady decrease in groundwater levels was again observed from June 2014 to date and are expected to continue the declining trend; The groundwater levels in this borehole have a slightly delayed response to rainfall, with recharge occurring approximately three months after high rainfall events

CONCLUSIONS: The groundwater levels in alluvium formation are sustained over long period following recharge events and losses of groundwater are mainly due to evaporative losses at near surface resulting in declining of groundwater level; The recharge of alluvium from Nyl River during periods of no flooding are probably confined to areas immediately adjacent to river; The groundwater levels in these boreholes have a slightly delayed response to rainfall, with recharge occurring approximately three months after high rainfall events; Over these two hydrological years (2014/15 and 2015/16), it is clear that during years of only limited or no floods, groundwater levels continue to decline.

Recommendations: The current groundwater monitoring in the floodplain needs to be reviewed for better coverage ; and Ensure alignment of the monitoring plan for the floodplain with the departmental monitoring strategy implementation plan.

Thank you