Table 3 should be here

3.2 Biochemical methods for denitriding

Fig. 3 showed the concentration distribution of O2, NO3- and DOC at typical moments of water level fluctuation under the conditions of different surface water DOC concentrations. In the surface water infiltration period (t  <1.5d), the concentration distribution of NO3- (or O2) was almost unchanged in all cases.  It was because the dominant factor affecting the concentration distribution of each solute was its infiltration capacity from surface water, by comparison, the change of DOC concentration in surface water had relatively small impact on AR, NI and DN during this period, namely, the solute reactivity was much smaller than the infiltration capacity. During the groundwater backflow period ( >1.5d), the concentration distribution of NO 3- (or O2) varied greatly under different cases, mainly due to the riparian zone no longer receiving the solute recharging from the surface water, and the chemical reaction was the dominant factor in the change of solute concentration distribution. Comparing the different cases in Fig. 3, with the increase of DOC concentration in surface water, the NO3 - plume shrunk obviously in the water level descending period, and the low-concentration-zone of O2 expanded significantly, which indicated that increasing the DOC concentration in surface water could strengthen the AR and DN and thereby enhance the denitriding effect. During the whole water level cycle, when the DOC concentration of surface water increased from 5mg/L to 15mg/L (namely, raise twice), the denitriding amount (M rem-NO3) increased from 6.34g to 22.70g (namely, raise 2.6 times), correspondingly, the maximum denitriding rate increased from 1.58g /d to 3.39 g/d. Since the total amount of NO3– infiltrating into the riparian zone (M in-NO3 =40.90g) was a constant, the denitriding efficiency (N rem-NO3) increased from 15.5% to 55.5 % (Table 4).
By comparing case 4, 5 and 6, the AR and DN processes in the riparian zone were obviously enhanced with the increase of DOC concentration in groundwater, which was corroborated by the decrease of overall O2 concentration, significant increase of low-value area of DOC concentration and obvious shrink of the NO3- ­plume during the water level descending period (concentration distribution picture was omitted). During the water level cycle, when the groundwater  DOC concentration increased from 0mg/L to 10mg/L, M in-NO3 increased from 6.34g to 22.23g, and the maximum denitriding rate increased from 1.58g/d to 4.89g/d correspondingly.M in-NO3 =40.90g kept no change, resulting in N rem-NO3 increased from 15.5 % to 54.5 % (Table 4). Thus, when the DOC concentration of surface water or groundwater increased to the same extent (10mg/L), the increase of maximum denitriding rate induced by surface water was smaller than that of groundwater (3.39g/d <4.89g/d), but the final denitriding amount was larger (22.70g >22.23g). This was because that the increase of groundwater DOC concentration could ensure the riparian zone with a relatively high DOC concentration in a short time, thereby improving the denitriding rate. However, the total DOC involved in the DN process was relatively small, resulting in a small denitriding amount.
Denitrifying bacteria mainly affected the DN process, while had little effect on AR. Comparing case 7, 8 and 9, the DOC and NO3- plumes involved in the DN process decreased with the increase of X DN, while the O2  plume had no significant change (concentration distribution picture was omitted). During the water level cycle, whenX DN increased from 2mg/L/d to 6mg/L/d , M rem-NO3 increased from 6.34g to 11.38g, the maximum denitriding rate increased from 1.58g/d to 4.61g/d correspondingly, while M in-NO3 =40.90g  kept no change, resulting in N rem-NO3 increased from 15.5% to 7.8% (Table 4 ). WhenX DN increased 2 times, the denitriding amount and maximum denitriding rate increased about 5g and 3g/d respectively. By comparison, when the surface water DOC concentration increased 2 times, the denitriding amount and maximum denitriding rate increased about 16g and 1.8g/d respectively. Therefore, increasing X DN could speed up the denitriding process, but the denitriding process would be limited to the infiltration amount of DOC from the surface water, causing the denitriding amount by increasing X DN was smaller than that by increasing the DOC concentration in surface or groundwater.