BARRIER effect of tunneling on groundwater flow in interbasin water transfer: case of northern collector tunnel, Kenya

Underground structures interfere with groundwater by seeping into the tunnel if it is a pervious structure or by interfering with the hydraulic head and flow direction if the structure is impervious. The latter effect is referred to as the barrier effect and results in groundwater level increasing upstream and decreasing downstream of the structure. The barrier effect is predicted to interfere with groundwater availability by creating an imbalance between areas upstream and downstream of the structure. To estimate the barrier effect of the Northern Collector Tunnel (NCT I) on groundwater flow in the Upper Tana basin, a vertical electrical sounding (VES) survey and MODFLOW modeling was carried out. VES was carried out using 4 profile lines (6 km long) along NCT 1 to identify groundwater recharge zones and characterize the aquifer. In addition, MODFLOW was used to estimate groundwater flows with a good simulation of R2 of 0.99. Results showed that many of the recharge zones are on the upstream side of the tunnel where many shallow springs of a maximum depth of 50m are located. Further, the study showed that the tunnel increased the hydraulic head on the upstream side by up to 10% with a 2% decrease observed in boreholes on the downstream side. This means that water users on the downstream side of the tunnel who rely on springs for their livelihood will be affected by the fall in water levels. Therefore, water managers and policymakers must implement measures to compensate the affected water users. This would mean exploring ecentralized water sources like rainwater harvesting to meet their water needs. In addition, groundwater levels should be continuously monitored along the tunnel to inform policy decisions. This will contribute towards achieving SDG goal number 6 by ensuring sustainable water management and access to water by all users.