Employee was bolting up a 24"" pipe in north bowlegs area, as he went to carry the impact gun to the flanges stepped into a wet area and his left foot went down about 8"" to 10"" into the mud. He went down at an angle and he felt a tingle in the top of his left foot as he fell to his right knee it twisted and his leg was bent underneath him and he felt pain in his right knee.