Credit: D. Scott Riegert
Ranney wells, which consist of a central concrete caisson, are designed to induce infiltration from a nearby surface-water source.
Ranney wells have come a long way since Leo Ranney installed the first horizontal collector well in the 1920s. These high-capacity wells offer an alternative to fields with many vertical wells.
Ranney wells comprise a central concrete caisson—typically 16 feet in diameter—excavated to a target depth at which well screens project laterally outward in a radial pattern. In a practice referred to as riverbank filtration, the wells are designed to induce infiltration from a nearby surface water source, combining the desirable features of groundwater and surface water supplies.
The result is an abundant, dependable supply of high-quality water with a constant temperature, low turbidity, and low levels of undesirable constituents such as viruses and bacteria. Riverbank filtration also provides an additional barrier to reduce precursors that might form disinfection byproducts during treatment.
Ranney wells have been designed with capacities from 2 to 80 mgd and cover a wide range of applications. Design options for the lateral screens are nearly endless, so the wells can be installed in various settings. In the right location, a Ranney well will produce the same volume of water as several vertical wells while using less area than a conventional well field. Additionally, a properly designed Ranney well has enough screens to minimize the entrance velocity of groundwater, reducing the frequency of required maintenance.
In the past, Ranney wells have been categorized by some state agencies as surface water sources because of their proximity to rivers and reliance on induced infiltration. Municipal water supplies that use Ranney wells designated as ground-water under the direct influence of surface water must decommission the wells, or upgrade treatment facilities and operator certifications to meet surface water treatment requirements. In most cases, upgrading a well presents operational and/or financial limitations the purveyor cannot overcome.
For example, the Long Term 2 Enhanced Surface Water Treatment Rule recently promulgated by the U.S. Environmental Protection Agency requires water sources with this designation to meet more stringent surface water treatment requirements, including two-log cycle removal of Cryptosporidium, three-log cycle removal of Giardia, and four-log cycle removal of viruses.
State regulators sometimes designated Ranney wells as “under the influence” using EPAcriteria. However, this practice is changing slowly as well owners have requested reviews of these designations. This process has involved evaluating site-specific aquifer conditions, a well's specific lateral intake configurations, and properly abandoning laterals with the potential to contribute surface water. Once a well's physical and hydrogeological design is found to meet the requirements for designation as groundwater, the investigation turns to the acquisition of extensive water quality data. If these data effectively demonstrate that a well is not susceptible to microbial contaminants from surface water sources, regulators may revisit its designation.
The city of Oxford, Ohio, is working closely with the EPA's district office to test, sample, and reconfigure their radial collector well #2. It had been taken offline 10 years ago after being designated a “surface water source.” When complete, this program will result in the state's redesignation of the radial collector well as a groundwater source.
— D. Scott Riegert is a senior hydrogeologist with Leggette, Brashears & Graham Inc., Columbus, Ohio; David D. Weihrauch is the water treatment plant manager for the city of Oxford, Ohio.