that sounds really nice LWJ, and welcome to PB. shorty and bobad give excellent advice. and as DD1 mentioned on yer other post, i would hope your water supply stays constant.

the most common way in my business that we estimate groundwater flow rate is conducting whats called a slug test, but you'd need a small PVC well screened across the water table near your pond site to do this.

you should be able to estimate a seepage velocity using a dye as well.

i'm sure there's other ways to do it, those just come to mind.

suffice it to say that if it is all clean sand (i.e. no silt) you could have conductivity values ranging from 1 to over 10,000 gallons/day/ft2.

GSF are people too!