Sampling Protocol
BTWD was specifically interested in three sample locations: Ivanhoe, NAPA Auto Parts and Lonnie’s Hydraulics. These sites receive water age ranging from three to seven days depending on the cycles and demands of the water system, with younger water typically coming directly from Jackson Station and older water typically associated with the system’s storage tank.
BTWD developed a sampling protocol to ensure that the sample sites adequately turned over water. The district also considered system demand and THM precursor variations and their impact on distribution THM readings.
The Ivanhoe and Lonnie’s Hydraulics sites are mainly residential neighborhoods influenced by outside watering and vacations that can vary system demands. NAPA Auto Parts, located in a predominantly commercial area, is influenced by the seasonal water demands of the regional middle school, which is the last customer before the sample point.
Well selection and groundwater hydrology were examined in more detail to determine their impact on THM formation. To aid in this review, the online analyzer was set up to analyze samples directly from the first customer in the distribution system. During plant operation, the analyzer would measure the TTHM levels at the end of the contact chamber, and during plant shutdown it would monitor flows heading back from the storage tank to the Ivanhoe sampling point.
Data Collection and Analysis
BTWD reviewed data points collected when the treatment plant was in operation. This data was then overlaid with well pumping data from the Air Force and Holden wells, which supply Jackson Station and are swapped on a biweekly basis. The data clearly depicted that the biweekly well switch had a significant impact on THM levels leaving the facility. The wells are in a semi-confined aquifer and the recharge mechanisms for the confined portions are slower and likely correlate to older groundwater, whereas the younger groundwater is likely recharged by surface waters.
Using the online THM analyzer, BTWD created THM system distribution simulation (THM-SDS) curves using production water. The THM-SDS curves were produced for each well source at day zero and day seven after the biweekly well change. These results supported the idea that the Air Force and Holden wells influence each other by shifting the zones of influence and subsequently the zones of contribution.
In early 2017, BTWD undertook a six-month pilot project to investigate new treatment techniques for the supplies at Jackson Station. The district ran the Holden well for two months, 24 hours a day. The online THM data from online and grab samples collected during this period (January and March 2017) indicated far more stable THM formation relative to the biweekly swapping of wells.
Lessons Learned
Until the online THM analyzer was installed, the data available were insufficient and the district was unaware of the significant fluctuations in THMs leaving Jackson Station. In this instance, the source of the fluctuation appeared to be the movement of the zone of contribution between the two production wells, which resulted in initial periods of high or low TTHMs.
Although BTWD’s research into the effects of well swapping and the creation of the THM-SDS curves left the district with a healthy questioning of what it can forecast within the distribution system, especially given the intermittent operation of the Jackson Station treatment facility, BTWD was nevertheless able to make some operational decisions based on the data collected in the field.
The district learned a significant amount of information from the samples processed by the online THM-100 analyzer. The data collected over the two-year THM monitoring study not only revealed a laboratory analysis error but also helped BTWD study and better understand supply and distribution management impacts on THMs, evaluate its analytical results and build the district’s confidence in the data collected. It also helped the district address its THM compliance concerns and optimize operations.
About the Author: Craig Douglas, P.E., is the assistant general manager for the Brunswick & Topsham Water District.
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