It would be difficult to comment on whether or not you have a septic problem without knowing more about the hydrogeology of your site(s). There are nutrient and bacterial issues. Pathogenic bacteria are removed relatively quickly in groundwater. If we knew how many days it took water to get from the tank to the pond we would know if there is a potential bacterial problem. The travel time between your drain field and pond is dependent on the the groundwater flow velocity as determined by soil permeability, porosity and hydraulic gradient. We don't know what these variables are and it would take some effort to figure it out. If its all gravel between the drain field and pond, you may indeed have a problem.
An easier way would be to pound a drive point to the correct depth about 25-feet from the pond and collect water samples for laboratory anlyses of fecal coliform and nutrients, which would cost a few dollars but maybe give you peace of mind. Nitrate alone is not adequate for wastewater. You would need to full nitrogen series nitrate, nitrite, ammonia, and TKD.
You may just be looking at iron bacteria, which creates an orange slime on the surface where groundwater seeps into the pond. It may also have an oily sheen. Remember groundwater typically does not contain oxygen. When it becomes aerated, iron is oxidized (ferric) creating a slimy mess that screws up our treatment systems, drains, etc. I can't comment on the water color or odor.
