Shallow alluvial groundwater wells are commonly used to extract surface water because they offer benefits such as water quality improvements, more consistent water quality, and decreased permitting requirements (compared to surface water diversions). The water quality improvements of alluvial wells are commonly known as Riverbank Filtration (RBF).
Shallow RBF wells face technical operations and maintenance (O&M) challenges including susceptibility to biofouling and limited production rates due to thin saturated aquifer thickness. These challenges are further exacerbated in areas where the surface water source for RBF contains treated wastewater effluent. Despite these O&M challenges, RBF wells in effluent dominated areas can be effective pre-treatment for indirect potable reuse (IPR).
Leonard Rice Engineers recently supported Castle Rock Water with expansion and enhancement of their alluvial well fields along East Plum Creek. This project improved existing well field yield by adding horizontal directionally drilled (HDD) wells. Directed Technologies Drilling provided horizontal drilling services to install HDD laterals near existing vertical wells. In most cases, pumping equipment was installed in the vertical wells. The HDD lateral wells were initially conceived as a way to improve the vertical well connection to the aquifer by having the HDD lateral pass very near the vertical well. In doing so, the wells performed better, doubling the original well yield and nearly quadrupling the yield immediately prior to rehabilitation and HDD lateral expansion.
During the project, LRE collected data to evaluate the performance of pumps placed in vertical wells versus horizontally in the HDD lateral wells. The results indicate that placing the pumps in the HDD lateral wells could increase the yield 2-3 times that of placing the pump in the vertical well. In other words, HDD lateral wells can increase the yield of pumps placed in vertical wells, but the system will produce even more water if the pump is installed in the HDD lateral wells. The vertical wells do not significantly increase the yield of the HDD lateral wells and aren’t a large contribution to the yield of these systems.
This expansion and enhancement project has resulted in higher yields and improved operations/maintenance for the Town’s alluvial RBF well fields. We drew several other conclusions about the planning, design and installation of HDD lateral wells:
- Comprehensive Site Investigation and Design – HDD laterals are ideally screened in more permeable materials that are sometimes difficult to identify in heterogeneous alluvial aquifers solely with investigation boreholes. A surface geophysical survey should be considered.
- Maximize Capture Area – If the HDD lateral cannot be installed across the creek or river, a parallel lateral installation will maximize the well’s capture area.
- Maximize Screen Length and Diameter – Reductions in material costs are a small savings compared to the benefits of increasing screen length and diameter.
- Pump Installation in HDD Lateral Well – As discussed above, an HDD lateral well does not benefit substantially from a vertical well. In cases were a vertical well already exists, HDD laterals can be beneficial in increasing yields at the vertical well.
These considerations can help with the successful design and construction of HDD lateral wells. HDD lateral wells can be beneficial in thin alluvial aquifers as new construction, or as expansion and enhancement of existing vertical wells and well fields.
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