The lawsuit filed last week was on behalf of Vernon and Nancy Greif, who live directly across the river from the project.
Attorney Erin Johnson, who is representing the couple, said commissioners failed to adhere to the county's land-use code in approving the project.
"They didn't act within their own regulatory framework," Johnson, of Rico, said Monday by phone.
County Attorney Sheryl Rogers could not be reached Monday for comment.
Property owner Lonnie Malouff has proposed to extract the soil over four years, after which he plans to create a series of ponds - seven in all - to serve as habitat for aquatic life and water fowl.
Last month, Commissioners Kellie Hotter and Joelle Riddle voted in favor of the project while Commissioner Wally White voted against it after an all-day meeting that included more than five hours of public testimony.
Some of the most ardent critics of the project were residents of homes in Goldeneye Estates, a subdivision where the Greifs live perched on the ridge opposite the Malouffs' property, which is one mile south of the Weaselskin Bridge on La Posta Road (County Road 213). They argued, as the lawsuit does, that the operation is incompatible with a rural residential neighborhood.
Proponents, who frequently referred to the family's long agricultural roots in the area, said the soil extraction operation is only temporary and when complete the land will be more beautiful than now.
After commissioners' Oct. 6 vote, Malouff said, "We still have property rights in La Plata County." Approval of the project included more than a dozen conditions, including the time limitation of four years, controls for dust and noise, and steps recommended by the Colorado Division of Wildlife.
The Greifs' lawsuit, which was filed in 6th Judicial District Court and seeks to have the decision overturned, argues that the county doesn't have the authority to enforce the four-year time limit.
"Any time limitation placed on (the permit) is unenforceable under state law and not an appropriate mitigating measure regarding the actual impacts of the proposed gavel mining operation," it reads.
Under Malouff's plan, soil and gravel will be extracted, sorted and screened on the property, but no crushing will occur on site.
The operation will be allowed to run 7 a.m. to 6 p.m. Monday through Saturday. Screening can occur from 7 a.m. to 5 p.m. weekdays and 7 a.m. to noon Saturdays.
Sale of the soil could fetch millions of dollars.
Upon completion, water from the Animas River would be diverted from the north end of the property, flow through a system of channels connecting the ponds and be released back into the river at the south end of the property.