The 2005 report on Sylvan Lake by AXYS Environmental estimated that about 0.5 tonne of Total Phosphorus and more than 1 tonne of Total Nitrogen nutrients enter Sylvan Lake each year from septic field seepage. Here is the Section 3.3.3. excerpt from that report:
Septic Field Performance Monitoring
The surficial materials dominating much of the area around the lake are not ideally suited to effective septic field operation. While new developments can be directed to employ communal collection systems to eliminate new risks from septic field installations, there are still a number of fields operating in the drainage basin, with little information on their performance and contributions to lake nutrient loading. During groundwater sampling conducted down gradient of septic fields for this study, wide ranging nutrient concentrations were measured among the wells, suggesting that septic field performance is variable around the lake.
Plenty of information is available to help keep your septic field operating at design specifications. Here are a few references: