Mitigation of Groundwater Dewatering Impact on a Nearby Contaminant Plume During Pipeline Construction
Presented on Monday, April 29, 2013
Bill Fernandez, PE1, John P. Nyznyk, PE, BCEE, LEEDAP2 and Robert P. Schreiber, PE, BCEE, D.WRE3, (1)Water Resources, CDM Smith, Walnut Creek, CA, (2)Environmental Remediation, CDM Smith, Walnut Creek, CA, (3)Water Resources, CDM Smith, Cambridge, MA
During the construction of water supply pipelines on a San Francisco Bay Delta island, dewatering wells and micro-tunneling operations were used to contend with the naturally high groundwater table on the island. The pipeline alignment passed within 1,000 feet of an existing methyl tertiary-butyl ether (MTBE) groundwater plume. Recognizing the potential for the dewatering operations to influence the MTBE plume, steps were taken to evaluate hydrogeologic conditions in the vicinity of the MTBE plume and to develop a mitigation strategy to prevent the dewatering operations from impacting the MTBE plume.
Evaluation of hydrogeologic conditions was based on updating an existing three-dimensional groundwater model developed as part of the preliminary design of the dewatering operations. The updated groundwater flow model was used to simulate groundwater conditions with and without the dewatering pumping to quantify the impact of the dewatering pumping on flow gradients near the marina site. The model was also used to evaluate the effectiveness of positioning a recharge trench between the MTBE plume and the dewatering wells as a way to overcome the influence of dewatering well operations on plume migration and to establish a hydraulic barrier to groundwater flow and MTBE plume migration.
A triangular network of monitoring wells was installed to establish a baseline groundwater gradient near the contaminated area before dewatering began. Baseline conditions also exhibited variation due to natural tidal influences from the nearby river. The rate of flow into the recharge trench was adjusted to maintain the groundwater gradients within an acceptable range based on the baseline conditions. Real-time modeling was used to confirm both dewatering and recharge system performance and to validate changes in recharge rates. The monitoring results indicated the recharge trench was effective in mitigating the impact on plume migration during dewatering operations.
Bill Fernandez, PE
Water Resources, CDM Smith, Walnut Creek, CA
Bill Fernandez is a water resources engineer with 12 years of experience in water resources system analysis and planning. He has extensive experience in performing modeling and analysis for simulation of groundwater flow and contaminant transport, optimization of water supply systems including supply wells, reservoirs, and raw water systems. Fernandez has developed a number of dewatering designs to address sensitive locations including dewatering in support of the excavation and construction of an earthen dam, dewatering next to sensitive wetlands, and dewatering control during pipeline construction in the vicinity of an active groundwater contaminant plume.
John P. Nyznyk, PE, BCEE, LEEDAP
Environmental Remediation, CDM Smith, Walnut Creek, CA
John Nyznyk has more than 26 years of experience in environmental engineering with a focus in site investigation and environmental remediation. He currently serves as the manager of the Environmental Remediation Division for CDM Smith in Northern California. His project work encompasses a broad range of engineering disciplines, including solid waste and hazardous waste management; planning, permitting, and design associated with remediation systems; hazardous waste site assessments; and environmental management program compliance.
Robert P. Schreiber, PE, BCEE, D.WRE
Water Resources, CDM Smith, Cambridge, MA
Robert Schreiber is a registered professional engineer with more than 39 years of experience in water resource planning and computerized engineering analysis. He graduated from MIT's Civil Engineering Department where he focused on groundwater hydrology and water resource systems analysis. He is a senior technical leader specializing in modeling of groundwater flow and contaminant fate and transport, and serves as a company-wide resource at CDM Smith. Schreiber was recently ASCE's alternate representative to the Federal Advisory Committee on Water Information, and is co-chair of its Subcommittee on Ground Water, focusing on implementation of a National Ground-Water Monitoring Network.
