from MJL Robin et al. 2003

EC is controlled by*:
1. Geology (rock types) – The rock composition determines the chemistry of the watershed soil and ultimately the lake or river. For example, limestone lead’s to higher EC because of the dissolution of carbonate minerals in the basin.
2. The size of the watershed relative to the area of the lake or river – A bigger watershed to river surface area means relatively more water draining into the river because of a bigger catchment area, and more contact with soil before reaching the river.
3. Other sources of ions to river or lakes – there are a number of sources of pollutants, which may be signaled by increased EC:
– Wastewater from sewage treatment plants (point source pollutants).
– Wastewater from septic systems and drain field on-site wastewater treatment and disposal systems (non-point source pollutants).
– Urban runoff from roads (especially road salt). This source has a particularly episodic nature with pulsed inputs when it rains or during more prolonged snowrnelt periods. It may “shock” organisms with intermittent extreme concentrations of pollutants, which seem low when averaged over a week or month.
– Agricultural runoff  [also runoff from large fertilized turfs] typically has extremely high levels of dissolved salts (non-point source of pollutants).
– A fraction of the total dissolved solids, nutrients (ammonium-nitrogen, nitrate- nitrogen and phosphate from fertilizers) and pesticides (insecticides and herbicides mostly) typically have significant negative impacts on streams or lakes receiving agricultural drainage water. If soils are also washed into receiving waters, the organic matter in the soil is decomposed by natural aquatic bacteria, which can [increase electrical conductivity and] severely deplete dissolved oxygen concentrations.
– Atmospheric inputs of ions are typically relatively minor except in ocean coastal zones where ocean water increases the salt load of dry aerosols and wet deposition. This oceanic effect can extend inland about 50-100 kilometers and can be predicted with reasonable accuracy.
4. Evaporation of water from the surface of a lake concentrates the dissolved solids in the remaining water – and so it has a higher EC.
– *From: Pilot Survey of Electrical Conductivity and Temperature in the South Nation River, Eastern Ontario, Michel J.L. Robin et al.,University of Ottawa 2003.