Spa Chlorine Granules (99% sodium dichlor) is the only  EPA registered sanitizer that has the ability to dissipate out of the spa water.  This product may also be used as a 'shock' and residual sanitizer, allowing bathers the option of not soaking in harsh chemicals or breathing toxic fumes.  To be more Environmental, ensure all the 99% sodium dichlor has dissipated out of the spa water before draining.  This can be done by leaving your cover open & running all pumps on high with all jets open for 30 minutes or until obtaining a 0 reading for both free & available chlorine with a spa test strip.  You can then drain your spa without harm to our ground water.