Nancy Correlli

Nancy Correlli, one of the co-owners of Baltimore Boating Center, stands in the parking lot, which is made of loose stone. She is holding a handful of stones from the lot. Baltimore County says the parking lot is impervious and is therefore subject to a $4700 rain tax. The county had previously declared that the lot was porous. (Baltimore Sun photo by Barbara Haddock Taylor / July 9, 2013)

Baltimore County business owners are criticizing the way the county has imposed its new stormwater management fee, saying officials miscalculated the amount of hard surface on their properties.

The complaints came as a county official acknowledged that the technology used to measure properties cannot tell the difference between surfaces that are impervious and those that are not.

The county used digitalized aerial photography from 2011 to measure building footprints and hard surfaces, said Vince Gardina, head of the county's Department of Environmental Protection and Sustainability. He said parking lots and roads are assumed to be impervious.

"The images are taken from a fairly high location," Gardina said.

Baltimore County is among the first jurisdictions to mail out bills assessing the new fee, dubbed the "rain tax" by detractors.

The state is requiring Maryland's nine largest counties and Baltimore City to collect the fee to help keep pollutants carried by stormwater runoff out of the Chesapeake Bay.

Some counties have balked at assessing the fee. Carroll County has opted to use grants and county dollars to fund bay projects instead of charging residents directly.

In Baltimore County, about 260,000 homeowners and businesses began receiving stormwater fee invoices as part of their property tax bills last week.

Baltimore City officials plan to mail theirs in the fall.

While homeowners in Baltimore County pay a flat fee that can range from $21 to $39, owners of commercial properties pay based on the amount of impervious surface — such as driveways, parking lots and roofs — on their land.

But critics say the county did not carefully calculate which surfaces count as impervious.

"There's no rationale to it," said Ron Gates, who owns a construction company and about 30 commercial properties in the county. "How are they coming up with these numbers to put on the bills?"

Gates said he owes a total of more than $30,000 for his properties. He spoke against the fee at a news conference Tuesday, appearing with Del. Pat McDonough at one of his buildings along an industrial stretch of North Point Boulevard in Dundalk.

McDonough recently launched a campaign to press fellow lawmakers to repeal the fee they passed in 2012.

The Republican delegate, who represents parts of Baltimore and Harford counties, said he is also working with Judicial Watch, a conservative foundation, to explore the possibility of a lawsuit against the state.

Nancy Correlli, co-owner of the Baltimore Boating Center, said she thought the stormwater fee was "a joke" when she first learned of it through a national television news segment. Then she received her tax bill, with a $4,700 stormwater fee.

"When I saw the bill, it wasn't funny anymore," said Correlli, who owns the marina with her family.

Loose stone covers most of the family's marina property along Sue Creek in Essex. She said county officials have told her previously that the stone is porous. She plans to appeal the fee.

Gardina said in most cases, gravel and stone are considered impervious because they can "become compacted and they don't allow water to run off." But he acknowledged that there are exceptions.

"The best approach for people who have these situations is to file an appeal, and we'll investigate it," Gardina said. "There's a way to use gravel in a design to allow for some infiltration, but it has to be designed that way. Most of these calls are coming from people who simply put gravel down."