Harrogate, like other famous spa towns, grew around the Georgian belief that certain mineralised spring waters, the fouler the better, offered health benefits. Although conferences have replaced spa waters as the town's visitor attaction, there are still places where the water can be sampled. Once such place is the Pump House (which also contains a small museum) where you can push a button to delivery a small quantity of water with a distinctive taste and smell.
To log this cache find the pump located on the outside wall of the pump house and get a picture of you drinking the water, and describe the experience in the log. Although convention dictates you should be photographed holding the GPS as well, this may not be possible if you need to cup your hands to drink the water, in which case it is acceptible to leave the GPS out of the picture.
Although there is no way to verify you have done this, honour demands you actually drink the stuff ;-) It is appearently good for you, and will certainly not cause you any harm, although it does repeat on you.