As others have written, good water and soil tests, maybe including a percolation test, are the first step. I wouldn't be so quick to add Epsom salt because you have no idea whether the excess salinity in the water, if any, comes from NaCl. Saline groundwater can contain a lot of different types of salt, including sodium chloride, sodium sulfate, sodium bicarbonate, magnesium sulfate (Epsom salt), calcium chloride, calcium sulfate, potassium chloride, and potassium sulfate, among a lot of others. I would be surprised if your water doesn't contain some calcium salts. How saline the water is and how much the salts accumulate in the soil are two important considerations. Of course, if the area has not been irrigated very long, there might not be a water-sourced salt buildup yet, even if your soil is susceptible to accumulation.
Working off the top of my head, I believe the reaction between NaCl and MgSO4 in pure water is: 2NaCl + MgSO4 <-> Na2SO4 + MgCl2, which I believe is reversible. Assuming it is reversible, I don't happen to know the equilibrium constant--maybe there's a 10-year old kid on the forum with a Gibbs free energy table who can whip out that Keq for us and correct any other errors here. Of course, the reaction under your conditions could be similar or very different, depending on the other salts in the system. The equation probably becomes a lot more complicated in an organic tea solution. One obvious point about that basic reaction is that it does not destroy the salt character of the system. No matter what the salt species are, a solution will begin to affect plants if the osmolarity becomes high enough.
If you are drinking that untreated water or giving it to your animals, you really need to know what's in it. One simple, not necessarily easy, way to reduce the salinity in the lawn/garden water is to dilute it with captured rain water. Other tactics for reducing hose-end salinity might include solar distilling and partial deionization, which might or might not be practical for large volumes depending on how much salinity you need to reduce, among others. Dilution probably is easier than elimination. You might be able to help the lawn/garden tolerance some by preferring more salt-tolerant species. Where you are and what type of soil you have can affect your choices for managing soil salt accumulation. Most of us here are of the opinion that organic methods will help your lawn and garden tolerate saline conditions better than synthetic management. Good luck.
_________________ In theory, theory and practice are the same; in practice, they aren't -- lament of the synthetic lifestyle.
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