Rising Chloride in a Coastal Well While the System Asks to Pump More
A community groundwater system draws from three wells in a sand aquifer about 1.8 miles from a tidal river. Well 2 is rated 700 gpm. Three years ago it produced water at 25 mg/L chloride and 30 mg/L sodium. Quarterly data now show 190 mg/L chloride and 105 mg/L sodium, rising about 5 mg/L chloride per quarter. Static water level has dropped 11 feet since 2019, and specific capacity has fallen from 8.0 to 4.6 gpm/ft. Well 2 is pumped 14 hours a day at 650 gpm into a 300,000-gallon clearwell blended with Wells 1 and 3, whose chloride has stayed at 22 and 30 mg/L. Blended finished water chloride is 105 mg/L. The system has asked DNREC to approve a new 400 gpm irrigation customer and to extend Well 2 to 18 hours a day. The last CCR reported sodium at 45 mg/L.
1. Distinguish the likely causes of the Well 2 trend and outline the investigations that would separate them.
Read the worked answer
Two different problems are landing on one well, and the tests to separate them are inexpensive.
Intrusion signature: chloride rising with sodium in parallel, roughly 5 mg/L per quarter, in a well 1.8 miles from tidal water, points to saltwater migration - either lateral movement toward the cone of depression or upconing from a deeper saline zone as drawdown increases. Rising chloride while capacity is still adequate supports this.
Well-deterioration signature: specific capacity falling from 8.0 to 4.6 gpm/ft alongside an 11-foot static decline can also mean screen incrustation (iron, manganese, carbonate), a worn or mis-set pump, or a gravel pack drawing fines. Reduced capacity forces longer pumping at higher drawdown, which then makes intrusion worse - the two can feed each other.
Investigations: a step-drawdown test to separate formation loss from well loss; a pump efficiency check and impeller wear inspection; downhole camera and caliper to look at the screen and casing; a chloride and specific conductance profile at 20-foot intervals while pumping, compared with a resting profile; a pump-off recovery test to see whether chloride falls when the well rests; and a review of nearby pumping, including the proposed irrigation well. Sample for bromide and strontium to fingerprint seawater versus connate brine.
2. How should you evaluate the chloride and sodium data against applicable standards, and what conditions would you attach to the expansion requests?
Read the worked answer
Standards. Chloride carries a secondary standard of 250 mg/L - a taste and corrosion threshold, not a health limit. Sodium has no MCL; EPA guidance is 20 mg/L for people on severely restricted sodium diets (500 mg/day), so sodium must be reported in the CCR and customer questions should be expected. Blended water at 105 mg/L chloride with reported sodium of 45 mg/L is within both today, but the trend, not the snapshot, is the issue.
The requests. Approving 400 gpm of new irrigation demand from a well already showing intrusion sends the wrong signal. A high-volume, low-return irrigation load increases annual withdrawal and can pull the cone of depression toward the river, and the request to run Well 2 six hours longer makes it worse. Both should be conditioned, not simply approved.
Conditions to propose: a chloride trigger, for example 120 mg/L at Well 2 or 150 mg/L blended, that automatically reduces Well 2 pumping; quarterly chloride, sodium, and conductance monitoring with an annual depth profile; metering and annual reporting for the irrigation account, with interruptible status during drought; a wellhead protection review of the area; and coordination with the state water supply planning process. If intrusion continues, evaluate blending limits, alternate sources, or membrane treatment, recognizing that RO or nanofiltration is capital- and brine-intensive for a three-well system.
3. Assuming the well stays in service, show how you would set an operating envelope and what blend control you would use.
Read the worked answer
Do the mass balance before changing hours. Chloride in the blend is the flow-weighted average: blended = (sum of Qi x Ci) / (sum of Qi). With Well 2 at 190 mg/L and Wells 1 and 3 near 22 and 30 mg/L: equal flows give about (190 + 22 + 30) / 3 = 81 mg/L. Running Well 2 at its 700 gpm rating against 300 gpm from the other two gives (700 x 190 + 300 x 26) / 1,000 = about 141 mg/L. That arithmetic belongs in the decision, before a schedule change, not after.
Operating envelope. (1) Set a permit or operating-plan limit on Well 2 hours per day tied to a blended chloride target (for example 150 mg/L) and to the sodium figure reported in the CCR. (2) Monitor chloride monthly at the clearwell outlet, not only at the well, and add online conductivity as a surrogate with an alarm. (3) Use the clearwell as a genuine blend tank - fill from all three wells together, not sequentially, so a single well cannot dominate the finished water. (4) Write a contingency: if the blend exceeds the trigger, cut Well 2 and make up volume from Wells 1 and 3, purchased water, or curtailment of the interruptible irrigation customer. (5) Re-run specific capacity and drawdown annually so a capacity decline is never misread as a blending problem.