I have not tried this, but believe modeling this would be difficult, and hard to verify if reliable. Some nutrients are more mobile in soils than others. Some soils and vegetation store more nutrients than others. The distance between septic discharge and stream is often sufficient to obscure discharge. You might try fluorescein dye in septic tank, and install activated charcoal packets in stream, removing a packet every few days or weeks and rinse in alcohol and test with fluorometer. Tom Aley USGS in Missouri used this approach to trace underground connections in karst topography. In addition most perennial streams are going to have substantial dilution potential in comparison to indirect discharge of septic wastes through soil, interflow or groundwater. If the septic field is full to capacity, with discharges obvious intermittently on the surface, the potential in combining with stormwaters and delivery to streams increases.

Dear William, Thank you for your response

Yes you can.