Re: New concept GM high density field
8 bar MIGHT be a bit to much pressure for your piping, depending on how you system is connected. would be to much pressure for glued joints but not for joints that are connected with say victaulic clamps. at 1.5 liters per minute your getting the job done by the sound of it. probably a good thing your throttling that motor down. at 5 bar per minute you would be doing over 70 psi which should be ok for your glued joints which I presume you are using.
your air leak is probably your shaft seal which might be wearing out from the dirty river water which should be very good for the plants. shaft seals can be pretty easy to replace depending on condition or type of pump. some fire pumps use a type of rope shaft seal that is very easy to replace. you just need a cork screw type puller. these seals require a lot of greasing. your pump should leak at the shaft seal if that is the problem.
if not your leak is from your suction pipe. I would watch out for your suction being to close to the surface of the water.
but I figure you already know this or have people who do. thought I would throw it out tho.