Brackish water is a mixture ...

Brackish water is a mixture of salt water and river water.  The total hardness is present from both of these sources and consists of Calcium and Manganese.  Not measured in the hardness but relevant is the sodium content.  The alkalinity, as Mr Amiri says, consists of hydroxide and carbonate.  Water sources rising in granite will have a low total alkalinity because the carbonate level is low but will have non-carbonate ions such as chloride, silicate and sulphate.  In brackish water the chloride level will be relatively high in proportion to the total hardness because it is in the form of sodium (salt).  The level of 517 microsiemens would suggest that proportion of water with high alkalinity water is low because sodium chloride rapidly increases the conductivity of the water.  A water from a "soft water" area will have a conductivity between 100-150 microsiemens and may only have a total alkalinity of between 10-30 ppm.  A harder water may have a conductivity of 350 -400 microsiemens and have an alkalinity of 150-200 ppm.   Some of this will be carbonate alkalinity while some will be non-carbonate alkalinity.  If a water comes from chalk for a conductivity of 400 microsiemens almost all of the total hardness will be bound to carbonate alkalinity.  So it depends on the water source flowing to the site where the brackish water forms.