Effects of tetracycline on water quality, soil and gases in aerated and unaerated leachfield mesocosms.

We examined the effects of tetracycline (TET) addition on the function of mesocosms representing aerated and unaerated septic system leachfields. Replicate mesocosms (n = 3) were filled with soil and either vented to a leachfield (LEACH) or aerated intermittently to maintain an O(2) level of approximately 0.21 mol mol(-1) (AIR). All mesocosms were dosed every 6 h for 10 d with 3 cm of domestic wastewater amended with 5 mg TET L(-1). Water quality parameters, headspace gas composition, and soil properties were measured prior to and during the dosing period, and for 42 days after the last antibiotic dose. No significant effect of TET was observed on the pH, level of dissolved O(2) or dissolved organic carbon (DOC) in drainage water from either treatment. In contrast, levels of Fe(2+) and SO(4) in drainage water from LEACH mesocosms decreased in response to TET dosing, with lower levels persisting until Day 52. Persistent increases were observed in the level of NO(3) in drainage water from AIR lysimeters and in NH(4) in LEACH mesocosms in response to TET additions. Removal of total P and DOC were unaffected by TET dosing in either treatment. Nitrogen removal in AIR mesocosms decreased during the TET dosing period, returning to pre-dosing values by Day 52. In contrast, TN removal in LEACH mesocosms increased during TET dosing, returning to pre-dosing values by Day 52. The composition of headspace gases in AIR mesocosms was not affected by tetracycline dosing. TET dosing resulted in significant increases in soil NH(4) concentration in LEACH mesocosms, whereas significant decreases were apparent in AIR mesocosms. Elevated levels of H(2)S and CH(4) in the headspace of LEACH mesocosms coincided with TET dosing and returned to pre-dosing levels when antibiotic dosing ceased. The effects of tetracycline on leachfield mesocosms differed as a function of aeration. Although most effects were transient, with values returning to pre-dosing levels after a 6-week recovery period in both treatments, persistent effects were observed in LEACH mesocosms.

Effects of tetracycline on antibiotic resistance and removal of fecal indicator bacteria in aerated and unaerated leachfield mesocosms.

Antibiotics can be present in low concentrations in domestic wastewater, but little is known about their effect on bacteria in onsite wastewater treatment systems. Mesocosms, consisting of soil-filled lysimeters representing the leachfield of a septic system under aerated (AIR) and unaerated (LEACH) conditions, were used to study the effects of tetracycline addition (5 mg L(-1)) to septic tank effluent on tetracycline resistance in the fecal indicator bacteria Escherichia coli and fecal streptococci, and on their removal. The mesocosms were dosed with antibiotic for 10 days, and effects monitored for 52 days. The fraction of resistant bacteria in mesocosm drainage water relative to that in septic tank effluent, GammaRes, for E. coli ranged from 0 to 0.66 in the AIR treatment and from 0 to 3.32 in the LEACH treatment. For fecal streptococci, GammaRes ranged from 0 to 0.41 and from 0.63 to 1.06 in the AIR and LEACH treatments, respectively. No significant differences in antibiotic resistance of fecal indicator bacteria were observed among sampling dates in soil or water from either treatment. Tetracycline had no significant effect on removal of fecal indicator bacteria, which ranged from 99.9 to 100% for E. coli and from 95.9 to 100% for fecal streptococci. Our results suggest that short-term addition of tetracycline at environmentally-relevant concentrations is likely to have minimal consequences on pathogen removal from wastewater and development of antibiotic resistance among pathogenic bacteria in leachfield soil.