Reclaimed Water for Groundwater Recharge: Quality Thresholds and Clogging Control from GB/T 19772-2005 to the Groundwater Regulation

1. The legal floor comes first

For groundwater recharge with reclaimed water, the first gate is legal, not water quality. The Regulation on Groundwater Management (promulgated by State Council Decree No. 748 on 2021-10-21, effective 2021-12-01, revised under the State Council decision of 2026-08-08 on amending and repealing certain administrative regulations) states in Article 43 that extraction and recharge of groundwater in multi-layer aquifers must prevent cross-layer contamination; where water quality differs greatly between layers, the layers shall be exploited separately; already contaminated phreatic and confined water shall not be mixed. The final paragraph of the same article is the direct basis for recharge: artificial recharge of groundwater shall comply with the relevant water quality standards and shall not degrade groundwater quality.

This has two implications: the recharge water must meet the relevant quality standard, and the recharge operation must not make groundwater worse. The second is an outcome test, not a compliance safe harbour.

2. The quality standard: two tiers in GB/T 19772-2005

The current national standard for recharge water quality is GB/T 19772-2005, Reuse of urban recycling water - Water quality standard for groundwater recharge. It gives basic control items and limits in two tiers by recharge method (surface spreading first, well injection second):

• color 30 / 15 dilution ratio; turbidity 10 / 5 NTU; pH 6.5 to 8.5;
• total hardness (as CaCO3) 450 / 450 mg/L; total dissolved solids 1000 / 1000 mg/L;
• sulfate 250 / 250 mg/L; chloride 250 / 250 mg/L;
• volatile phenols (as phenol) 0.5 / 0.002 mg/L; anionic surfactants 0.3 / 0.3 mg/L;
• chemical oxygen demand (COD) 40 / 15 mg/L; five-day biochemical oxygen demand (BOD5) 10 / 4 mg/L;
• nitrate (as N) 15 / 15 mg/L; nitrite (as N) 0.02 / 0.02 mg/L;
• ammonia nitrogen (as N) 1.0 / 0.2 mg/L; total phosphorus (as P) 1.0 / 1.0 mg/L;
• animal and vegetable oils 0.5 / 0.05 mg/L; petroleum oils 0.5 / 0.05 mg/L;
• cyanide 0.05 / 0.05 mg/L; sulfide 0.2 / 0.2 mg/L; fluoride 1.0 / 1.0 mg/L;
• fecal coliform 1000 / 3 per L.

Note a to Table 1 adds that the surface clay layer should not be thinner than 1 m; where it is thinner, the well injection requirements apply. This is an easily missed escalation clause: if the site does not qualify, the limits automatically tighten to the stricter tier.

Selective control items (Table 2) include total mercury 0.001 mg/L, alkyl mercury not detectable, total cadmium 0.01 mg/L, hexavalent chromium 0.05 mg/L, total arsenic 0.05 mg/L, total lead 0.05 mg/L, total nickel 0.05 mg/L, total beryllium 0.0002 mg/L, total copper 1.0 mg/L, total zinc 1.0 mg/L, total manganese 0.1 mg/L, total selenium 0.01 mg/L, total iron 0.3 mg/L, total barium 1.0 mg/L and benzo(a)pyrene 0.00001 mg/L.

3. Subsurface residence time: surface and well recharge differ by a factor of two

Clause 4.3 requires recharge water to remain underground long enough before extraction, to further destroy pathogenic microorganisms and ensure sanitary safety. Clause 4.3.1: for surface spreading, the water shall remain underground not less than 6 months before extraction. Clause 4.3.2: for well injection, not less than 12 months.

This clause directly determines the layout of extraction wells and the pumping schedule. Well injection has a short flow path and limited aquifer purification, so a longer residence time compensates. When locating extraction wells, the hydraulic residence time must be checked against 4.3.1 or 4.3.2; water quantity alone is not sufficient.

4. Recharge clogging: three mechanisms, and the standard closes only one entrance

Physical clogging

Suspended solids, turbidity and entrained gas accumulate at the infiltration surface or at the well screen and reduce recharge capacity directly. GB/T 19772-2005 controls physical clogging through turbidity: not above 10 NTU for surface spreading and not above 5 NTU for well injection. Well injection is twice as strict because the flow cross-section is smaller and recovery after clogging is harder.

Chemical clogging

Mixing of recharge water with formation water triggers water-rock interaction; oxidation and precipitation of iron and manganese and deposition of calcium carbonate all reduce porosity. The standard constrains this indirectly through iron, manganese and total hardness: total iron 0.3 mg/L, total manganese 0.1 mg/L (selective control items in Table 2) and total hardness 450 mg/L. Note that iron and manganese are selective items, so whether they become mandatory tests depends on site hydrogeology.

Biological clogging

Microbial growth at the infiltration surface forms biofilm and is one of the most common causes of recharge capacity decline. The standard constrains it through organic matter and microbial items: COD and BOD5 limit the bioavailable substrate, and fecal coliform (1000 / 3 per L) limits the pathogenic risk. Dissolved oxygen is not limited in Table 1 of GB/T 19772-2005, but it governs aerobic bacterial growth and should be observed during operation.

What the standard does not cover

GB/T 19772-2005 does not specify recharge rate, backwashing (redevelopment) interval, redevelopment method or per-well recharge volume. These depend on aquifer hydraulic conductivity, well construction and recharge water quality, and can only be established by field hydrogeological testing. Assigning fixed values to them is unsupported.

5. How far is the gap from Grade 1A effluent

Placing GB 18918-2002 Grade 1A alongside the recharge limits makes the gap obvious. Using a comparison published in a local reclaimed water master plan (shown as recharge limit / Grade 1A): color 15 / 30 degrees; turbidity 5 NTU / not specified; pH 6.5 to 8.5 / 6 to 9; total hardness 450 / not specified; dissolved oxygen not less than 1.0 / not specified; BOD5 4 / 10 mg/L; COD 15 / 50 mg/L; ammonia nitrogen 0.2 / 5 mg/L; nitrite 0.02 / not specified; total dissolved solids 1000 / not specified; mercury 0.001 / 0.001; cadmium 0.01 / 0.01; arsenic 0.05 / 0.1; chromium 0.05 / 0.1; lead 0.05 / 0.1; iron 0.3 / not specified; manganese 0.1 / 2.0; fluoride 1.0 / not specified; cyanide 0.05 / 0.5; fecal coliform 3 / 1000 per L.

The plan concludes explicitly that Grade 1A effluent falls far short of GB/T 19772-2005 and is therefore not considered for groundwater recharge. The three largest gaps are ammonia nitrogen (0.2 versus 5), COD (15 versus 50) and fecal coliform (3 versus 1000), a factor of 25, 3.3 and 333 respectively. Ammonia nitrogen and fecal coliform cannot be closed by conventional secondary treatment plus disinfection; targeted advanced treatment such as further nitrification and membrane filtration is required inside the water reclamation plant.

A second reference frame is GB/T 14848-2017, Standard for groundwater quality (issued 2017-10-14, effective 2018-05-01, replacing GB/T 14848-1993). Its Class III limits include turbidity not above 3 NTU, total hardness not above 450 mg/L, total dissolved solids not above 1000 mg/L, sulfate not above 250 mg/L, chloride not above 250 mg/L, iron not above 0.3 mg/L, manganese not above 0.10 mg/L, fluoride not above 1.0 mg/L, cyanide not above 0.05 mg/L, permanganate index not above 3.0 mg/L, ammonia nitrogen not above 0.50 mg/L, nitrate not above 20.0 mg/L, nitrite not above 1.00 mg/L, total coliform not above 3.0 MPN/100mL and total bacterial count not above 100 CFU/mL. Whether recharge has "degraded" the aquifer is ultimately judged against this standard.

6. Implementation checklist

1. Confirm the site: whether the surface clay layer is at least 1 m thick, which decides whether the surface or the well injection tier applies.
2. Confirm the legal basis: Article 43 of the Regulation on Groundwater Management requires prevention of cross-layer contamination and prohibits degradation of groundwater.
3. Check every item in Table 1 of GB/T 19772-2005, focusing on ammonia nitrogen, COD and fecal coliform.
4. Complete a full baseline survey of the Table 2 selective control items before recharge starts.
5. Verify subsurface residence time (6 or 12 months) under clauses 4.3.1 / 4.3.2 and lay out extraction wells accordingly.
6. Determine recharge rate and redevelopment interval by field hydrogeological testing; do not copy empirical values.
7. Monitor aquifer quality against GB/T 14848-2017 after recharge begins, to verify the "no degradation" outcome test.

Limits in this article are taken from Tables 1 and 2 and clause 4.3 of GB/T 19772-2005; the legal basis is the Regulation on Groundwater Management (State Council Decree No. 748, 2026 revision); aquifer assessment is based on GB/T 14848-2017 (effective 2018-05-01). The Grade 1A comparison is quoted from a publicly released local reclaimed water master plan and is indicative of the order of magnitude of the gap.