When a water authority doesn't report it that generally means that it isn't too significant.
All I can do is make the following assumptions
1. The 88 ppm is as Ca++ (not ppm as CaCO3)
2. The alkalinity titration was done to pH 4.3
3 There are no ions in the water except Ca++, Mg++, Cl-, Na+, HCO3-, CO3-. SO4--, H+ and (OH)-.
Based on that your magnesium is 13.4 mg/L.