Granite Sluiceway Waterproofing

[thunberg]

Not sure if this is the right forum to post this in, but I appreciate any help or direction anyone may be able to offer.

I own a 19th century Greek Revival former woolen mill located in Southern RI, and have been in the process of restoring it over the past few years. There's a dam, gatehouse, and associated sluiceway on the property, and one of the nagging issues is that before I purchased the property, a section of the sluiceway was rebuilt with granite masonry walls that leak quite badly.

The property is here -- Google Map -- Street View

Note that I'm currently diverting water upstream and the sluiceway is currently 2-3' below normal level, as can be seen by the algae line.

When the water level is normal, there's enough water getting through, or under, that the parking lot has a steady stream in it, which can be seen here via Google Maps:

Photos:














I've talked to quite a few people about this and would like some additional input on the following options as I'm most likely missing something:

1. Rebuild the wall using hydraulic mortar, as apparently this wasn't used in the original construction.

2. Chip out the interior mortar, and repoint with hydraulic mortar.

3. Spray the interior of the wall with a dark gunite.

4. Varying forms of granite / masonry sealer.

I'm not entirely concerned with cost, more doing it right in the most watertight and permanent way possible so that this isn't an ongoing maintenance issue. Thoughts?