Trinity Aquifer

The Trinity Aquifer is composed of coastal plain units. In order, these include Pike Gravel, Delight Sand, Dierks Limestone, the Holly Creek Formation, the De Queen Limestone, and Paluxy Sand. There are three sands that are important for the aquifer’s permeability: Pike Gravel, the Ultima Thule gravel member of the Holly Creek formation, and the Paluxy Sand formation.

This aquifer is dated to the early Cretaceous period, about 145 million years ago. Its geologic units were deposited by the movements of an ancient sea, hence the “coastal plain” designation.

The Trinity Aquifer extends geographically across southwestern Arkansas. Its physiographic province is designated as Coastal Plain. The aquifer has both confined and unconfined sections.

USGS Aquifer codes: 218TRNT

The Trinity Aquifer has both confined and unconfined sections. The thickness of the aquifer fluctuates in different locations; it ranges from a few tens of feet to greater than 1,000 feet. The water table depth of this aquifer typically hovers in the 50 to 800-foot range but jumps to 3,000 feet in some areas.

This aquifer is sourced by rainfall on outcrops and recharge into faults or cracks. According to USGS, the wells commonly yield from 50 to 200 gallons per minute with a maximum of 3,000 gallons per minute. The hydraulic conductivity varies from 1 to 31 feet per day.

The Trinity Aquifer’s transmissivity ranges from 80 to 5,700 square feet per day. The storage coefficient ranges from about 2x10-5 to 0.026.

There are management concerns regarding aquifers in general which would apply to the Trinity Aquifer. Some believe we should be utilizing more surface water in order to preserve the groundwater stored in aquifers. The Trinity Aquifer is noted to maintain low salinity, as supported by the chloride present in the aquifer. Both nitrate and iron levels are noted to be particularly low, as well. In contrast, there is a spike in the aquifer’s sulfate. All of these should be at safe levels according to EPA standards.