Innovative Techniques for Targeted Infiltration and Inflow Investigations

Traditional methods of infiltration and inflow (I/I) assessment are based on catchment flow monitoring and analysis against standard I/I parameters. Such studies provide the basis for I/I remediation programmes that are often targeted at the catchment level which may contain kilometres of pipe. An opportunity was identified to provide much more value to the I/I investigations in the township of Levin.

Horowhenua District Council (HDC) had experienced major operational and environmental issues resulting from excessive wet weather flows and elevated base flows during winter. During the course of investigations groundwater infiltration (GWI) was identified as the main contributing factor and the problems were isolated to the catchments in the north. The extent of GWI in the identified area was such that over 140,000m3 of was entering annually which represented over 90% of the total I/I.

Locating the source of GWI became the primary focus and groundwater (GW) submergence analysis and night flow isolation studies were carried out that allowed sourced of GWI to be identified in lengths of 100-600m. The key outcome was isolating 89% of the total I/I from the northern catchments to 1806m of pipe representing less than 2% of the total Levin Network. GWI from this small length of pipe was responsible for 9% of the annual flow to the treatment plant. The source of historical extreme GWI could also be determined using the submergence analysis tool and taking into account other contributing factors such as pipe material and age. The combination of methods allowed a prioritised I/I rehabilitation plan to be developed so that future remedial works could be applied in a targeted cost effective way.