Table of Contents Next Section
The New Jersey Department of Health and Senior Services (NJDHSS), with support from the Agency for Toxic Substances and Disease Registry (ATSDR), is conducting an epidemiologic study of childhood leukemia and nervous system cancers that occurred in the period 1979 through 1996 in Dover Township, Ocean County, New Jersey. The epidemiologic study is exploring a wide variety of possible risk factors, including environmental exposures. ATSDR and NJDHSS have determined that completed human exposure pathways to groundwater contaminants have occurred in the past (through private and community water supplies) in some parts of the community. To investigate this exposure, ATSDR developed a water-distribution system model specific to the Dover Township area using the EPANET 2 software. Results obtained from the model—the percentage of water derived from different sources that historically supplied the water-distribution system—are considered one of the risk factors in the epidemiologic investigation.
The first step of the analysis was to calibrate the model to present-day (1998) water-distribution system characteristics using hydraulic and system-operations data collected during March and August 1998. Results of the 1998 field-data collection activities and model calibration were described in a previous ATSDR report. The second step of the analysis, and the subject of this report, was the application of the calibrated model to simulate operations during the historical period of January 1962 through December 1996. Hydraulic and source-trace analysis simulations were conducted for each month of the historical period (420 months) using EPANET 2. Results of these model simulations are reported herein in terms of the percentage contribution of water from distribution system points of entry (wells and well fields) to locations throughout the Dover Township area. Seven representative years are discussed in detail—1962, 1965, 1971, 1978, 1988, 1995, and 1996.
Analysis of water production data indicated that the historical water-distribution system could be characterized by three typical demand periods each year: (1) a low- or winter-demand period, generally represented by the month of February and designated herein as the minimum- demand month; (2) a peak- or summer-demand period, represented by one of the months of May, June, July, or August and designated herein as the maximum-demand month; and (3) an average-demand period, generally represented by the month of October and designated herein as the average-demand month. The historical production data indicate that considerable production increases occurred in 1971, 1988, and 1995.
To simulate the distribution of water for each of the 420 months of the historical period, network configuration, demand, and operational information were required. Before 1978, operational data were unavailable. To compensate for this lack of critical information, system-operation criteria were developed, and designated as the "Master Operating Criteria." These criteria are based on hydraulic engineering principles necessary to successfully operate water-distribution systems similar to the one serving the Dover Township area. From 1978 forward—for selected years—operators of the water utility provided descriptions of generalized operating practices for a typical "peak-demand" (summer) and "non-peak demand" (fall) day. These guidelines were used in conjunction with the "Master Operating Criteria" to simulate a typical 24-hour daily operation of the water-distribution system for each month of the historical period.
For the period of the investigation, the physical characteristics and potable water production capacity of the distribution system changed considerably. In 1962, the water-distribution system served nearly 4,300 customers from a population of about 17,200 persons. As characterized for modeling purposes, the water-distribution system consisted of: (1) approximately 2,400 pipe segments ranging in diameter from 2 to 12 inches; (2) a total service length of 77 miles; (3) three groundwater extraction wells with a rated capacity of 1,900 gallons per minute; and (4) one elevated storage tank and standpipe with a combined rated storage capacity of 0.45 million gallons. Annual system production was 359 million gallons which included the production of about 1.3 million gallons per day during the peak-production month of May.
By contrast, in 1996, the last year of the historical reconstruction period, the water-distribution system served nearly 44,000 customers from a population of about 89,300 persons. As characterized for modeling purposes, the water-distribution system consisted of: (1) more than 16,000 pipe segments ranging in diameter from 2 to 16 inches; (2) a total service length of 482 miles; (3) twenty groundwater extraction wells with a rated capacity of 16,550 gallons per minute; (4) twelve high-service or booster pumps; and (5) three elevated and six ground-level storage tanks with a combined rated capacity of 7.35 million gallons. Total annual system production was 3,873 million gallons which included the production of about 13.9 million gallons per day during the peak-production month of June.
In order to simplify the rigorous data requirements needed to simulate the historical water-distribution systems, a surrogate or alternative method, designated herein as the "supply-node-link" or SNL simulation method, was devised. Using this method, balanced flow conditions were maintained and the measured volumes of monthly water production were used while avoiding the need for detailed network operations data, which were not available for most of the historical period. Comparison of flow results obtained using the surrogate SNL simulation method with measured flow data obtained during August 1998 for the Holly and Parkway treatment plants showed that the SNL method simulated nearly identical flows to those measured.
Simulation of the proportionate contribution of water from wells and well fields to selected network locations in the Dover Township area, was accomplished using the trace-analysis option of EPANET 2. Proportionate contribution simulations were accomplished using the "Master Operating Criteria" and manual adjustment of model parameters. The parameters adjusted were the on-and-off cycling patterns of wells and the operational extremes of water levels in the storage tanks. This modeling approach was designated the "manual adjustment process." In addition, the assumption was made that a one-month period of operations could be reasonably represented by a "typical" 24-hour day for each month of the historical period.
Proportionate contribution simulations conducted using the manual adjustment process illustrate the increasing complexity and operational variability of the distribution system throughout the historical period. Simulation results for the maximum-demand months of May 1962, June 1965, July 1971, June 1978, July 1988, August 1995, and June 1996 for a pipeline location in southeastern Dover Township (designated herein as pipeline location D) exemplify the annual variation in the contribution of water to this location and indicate the following:3
To address the issue of uncertainty and variability of system operations, and specifically to test the sensitivity of the proportionate contribution results to variations in model-parameter values, a set of alternate operating conditions different from those determined using the manual adjustment process were developed and tested. Alternate operating conditions were simulated using a Genetic Algorithm (GA) optimization approach and were also required to satisfy the "Master Operating Criteria" and to result in the satisfactory operation of the historical water-distribution system. Four sets of hydraulic and operational constraints were considered for variation and analyses in order to determine the effects of parameter variation on the simulated proportionate contribution results. The constraints subjected to variations were: (1) pattern factors assigned to wells and supply nodes, (2) minimum pressure requirements at model nodes, (3) allowable storage tank water-level differences between the starting time (0 hours) and ending time (24 hours) of a simulation, and (4) daily system operations represented by a "typical" 24-hour day over a month-long period. For the first three types of constraints, GA optimization methods were used to determine sensitivity analysis results for the proportionate contribution of water at all pipeline locations. These results were compared with results previously obtained using the manual adjustment process. For the fourth type of constraint variation, the manual adjustment process was used to obtain simulation results for the sensitivity analysis.
Sensitivity analysis results indicate small variations when comparing the proportionate contribution results from the manual adjustment process to results obtained using GA optimization methods. Analyses of differences in the simulation results show that the simulated proportionate contribution of water from wells and well fields is relatively insensitive to changes in system hydraulic and operational constraints. For a 24-hour period, the average percentage of water over all study locations derived from all wells or well fields using either the manual adjustment process or any of the GA simulations does not vary appreciably. Statistical analyses of the differences in simulated proportional contribution results obtained using the manual adjustment process and the sensitivity analyses show that differences are normally distributed for study locations, and that, overall, the difference distributions were characterized by a mean, mode, and median of nearly 0% and a standard deviation of less than 4%. As a consequence, minor differences in the simulated proportionate contribution of water between the manual adjustment process and the sensitivity analyses indicate that there was a narrow range within which the historical water-distribution system could have successfully operated to maintain a balanced flow condition and satisfy the "Master Operating Criteria".
To test the validity of the assumption that daily system operations over a period of one month could be represented by a "typical" 24-hour day for each month of the historical period, additional sensitivity analyses were conducted using hourly operational data obtained from the water utility for 1996. Month-long simulations were conducted for February, June, and October 1996 which represented, respectively, the minimum-, maximum-, and average-demand months. When results for the month-long simulations (averages over the monthlong period) were compared with results from the "typical" 24-hour day, differences in simulated proportionate contribution of water to five pipeline locations—designated A, B, C, D, and E—were small. As an example, for June 1996, the difference in the contribution of water from the Parkway well field for the two methods of simulating the daily system operations were 0% for location A, 1% for location B, 4% for location C, 2% for location D, and 3% for location E. Therefore, sensitivity analysis assisted in confirming that the day-to-day operations of the water-distribution system were highly consistent over a month-long period (based on available 1996 hourly data) and could be represented by a "typical" 24- hour operational pattern
The sensitivity analyses conducted as part of the historical reconstruction of the water-distribution system serving the Dover Township area indicate that: (1) there was a narrow range within which the historical water-distribution systems could have successfully operated and still satisfy hydraulic engineering principles and the "Master Operating Criteria," and (2) daily operational variations over a month did not appreciably change the proportionate contribution of water from specific sources when compared to a typical 24-hour day representing the month.
Overall, the simulation results for the proportionate contribution of water from wells and wells fields indicate variation by time and location. However, the results also show that certain wells provided the predominant amount of water to locations throughout the Dover Township area. The reconstructed historical water-distribution systems and applied operating criteria—based on the "Master Operating Criteria" and using generalized water-utility information—are believed to be plausible and realistic scenarios under which the historical 1962–1996 water-distribution systems were operated.