Our pond recently started leaking again and I finally figured out what the issues are. We have a 1.5 acre pond. we cut the dam down 5 feet last year and widened it about 15 yards with the excess dirt. The pond is 40 years old or more and was built in a very rocky area. the area to the right of the dam has some spots that are opening up like mini sink holes and water is just draining straight out It doesn't appear to be ending up behind the dam anywhere it is just going straight down into the earth. I have been filling the holes with concrete as I find them and found 4 more over the weekend so I filled the holes were hundreds of pounds of concrete. My question is would it be better to rent an excavator and dig out the 20 yard area or just hire a concrete truck to come in when it dries out and dump concrete on top of the ground to seal off the holes that keep opening up? It appears to be isolated to around a 20 yard bank that goes up to the dam. One of the holes was the size of a basketball and draining the pond down 4 inches over night.

thanks for any advice,