Decreasing pH Level of Siltation Ponds

How can I decrease the pH of siltation ponds because we are currently discharging water of the pH around 11 from the underground. The natural pH of the water in the area is around 8.5 - 9 due to the geology of the area (alkaline porphyry). I tried dosing dry acid but it costs a lot. Does anyone have a more economical solution?