At Monday night’s City Council forum, South Ward candidate Curt Winston asked his fellow council hopefuls whether or not a moratorium on large-scale developments should be considered until the release of a United States Geological Survey groundwater study next spring.

The following day, members of the USGS presented the study’s parameters to 30 concerned citizens and city officials about the variables involved, the health of the island’s aquifers and more.

The model is a complex mathematical computational tool that can assess how a multitude of factors will affect the future island water supply. It includes the impacts of more water usage, growth of impervious surfaces on groundwater recharge and stream vitality, and even hypothetical scenarios relating to dramatic changes in weather patterns.

The results of the model will characterize how much groundwater is available, where it comes from and where it goes. Island residents draw their freshwater from five aquifers.

Matt Bachmann of the USGS said the success of the model is heavily dependent on accurate predictions of future conditions.

“If we build a model using 3 percent growth and it turns out to be 7 percent, then the model doesn’t work and I look bad,” Bachmann said.

City Long Range Planning Manager Libby Hudson said the city is in the process of developing scenarios of high growth, low growth and most-likely growth to give to the USGS.

Multiple scenarios are needed because it’s impossible to correctly project growth rates 20 or 30 years into the future. Island population grew dramatically in the 1990s, but has slowed in recent years, including an increase of about 100 people during each of the last two years.

“It’s a prediction; it’s a best guess based on what’s occurred in the past,” she said.

Hudson said the projections go beyond 2025, the year a citizen advisory committee created a projected population of 28,660. The model can be adjusted throughout the process if council policies alter water usage, growth or development.

Councilor Barry Peters asked the experts at Tuesday’s meeting whether potential policies like an altering of water bills to take money off the flat fee and weigh it more on usage, or the Housing Design Development Project (HDDP) could jeopardize the model’s efficacy.

“If it can be translated to a reduction or increase in water usage then the model can handle it,” said USGS hydrologic expert Gary Turney.

A previous groundwater study performed in 1988 by the USGS concluded that Bainbridge’s groundwater was suitable for most uses and the majority of samples passed state standards for drinking water. Though not a major water-quality issue according to the report, of the 48 samples taken for the study, 19 contained an excess of manganese and three contained too much iron.

The study was able to judge the quality of water in 1988, but the report could not provide any information on the effects of further groundwater development without the creation of a complex mathematical model.

The current study, which began in 2006, is based on data from more than 100 wells throughout the island. The southern portion of the island receives less monitoring because of the underground layer of bedrock it possess.