GB/T 43932-2024Guidance for the monitoring of the karst carbon cycle and the evaluation of enhanced carbon sink in karst catchment (English PDF)

3 Terms and definitions

A karst catchment is defined as a complete catchment unit of a surface or underground water system in which the area of carbonate rock is not less than 10 %, the bicarbonate concentration of the groundwater or surface water body present is not less than 1 mmol per litre, and the hydrochemical type is the bicarbonate type.

The karst carbon cycle is defined as the process in which, within a karst catchment, the weathering and dissolution of carbonate rock consumes carbon dioxide from the atmosphere or the soil and forms inorganic carbon, the forms of carbon then converting into one another and migrating under physical, chemical and biological action. The karst carbon sink is the process, activity or mechanism by which carbon dioxide is absorbed from the atmosphere or the soil through the karst carbon cycle and stored in the water body.

The term for the artificial enhancement of the sink is printed in the document as artificail carbon sink increasing, and is defined as the process by which activities such as artificial afforestation and grass planting, soil improvement, the introduction of allogenic water and the cultivation of aquatic plants promote the absorption of more carbon dioxide from the atmosphere or the soil by karst processes.

4 General provisions

4.1 The work surveys the geological and hydrological structural features of the karst catchment and monitors the source, migration, conversion, form of occurrence, distribution features and flux of carbon in the karst carbon cycle. It builds a database of the carbon cycle and of sink enhancement in karst areas, prepares maps of the karst carbon cycle and of carbon sink effects, assesses the source and sink effects of the cycle, computes the karst carbon sink intensity of typical catchments and carries out the evaluation of that intensity and of its enhancement. It analyses in particular the controlling factors of the karst carbon sink in the monitored area and selects the best techniques for enhancement, so as to serve national and local needs.

4.2.1 and 4.2.2 On the basis that the general pattern of the working area is grasped, the work is preferably carried out first in regions where regional geological, hydrogeological or environmental geological surveys have already been made. According to the exposure conditions and the features of karst development, the karst areas of China are divided into the southern karst area, the northern karst area and the Qinghai-Tibet plateau area, whose distribution and features are given in Annex A. In the southern karst area the recharge, runoff and discharge conditions of groundwater, the hydrodynamic features and the hydrochemical regularities are relatively complex, the regional hydrogeological conditions change markedly and ecological problems stand out, so that it belongs to the complex class, the other two areas belonging to the simple to moderate class. The main technical quotas of the survey follow GB/T 14158, GB/T 12719 and Annex B.

4.2.3 The survey and monitoring proceed by catchment. Catchments are divided into four classes by area: not greater than 20 square kilometres; greater than 20 and not greater than 500 square kilometres; greater than 500 and not greater than 10 000 square kilometres; and greater than 10 000 square kilometres. The monitoring indicators are settled according to the class of catchment, as set out in Annex C.

4.2.4 and 4.2.5 Full use is to be made of historical material; existing work that meets the quality requirements may be counted toward the technical quota, further work then being deployed as needed. In areas where the degree of hydrogeological study is low, the workload of survey points follows Annex B; where it is moderate, 70 % of that workload is deployed; where it is high, 50 %. Table 1 divides the survey area into three zones by degree of hydrogeological study: the zone of high study, where a dedicated hydrogeological survey at a scale of 1 to 50 000 or larger has been made, where in mountainous ground a regional geological survey at that scale or larger has been made, and where in plain and basin ground a Quaternary geological survey has been made; the zone of moderate study, where a regional hydrogeological survey at a scale of 1 to 100 000 or 1 to 200 000 has been made, or a hydrogeological investigation at a larger scale; and the zone of low study, a blank area for regional hydrogeological work, or one where only a small-scale regional hydrogeological survey below 1 to 200 000 has been made.

4.2.6 and 4.2.7 Routine water quality analysis comprises the simplified analysis and the full analysis. The number of routine water quality samples preferably makes up 30 % to 50 % of the hydrogeological points, that is machine wells, dug wells, springs and surface water bodies, of which full analysis samples preferably reach 10 % to 20 % of the water quality samples; full analysis, pollution analysis and isotope analysis samples for monitored water points may be arranged as needed. Where the survey area contains special ground such as high mountain gorges, plateau permafrost, nature reserves or military control areas, the technical quota may be settled according to the real situation, the minimum monitoring density of Annex B being taken as the reference.

5 Work flow

The work flow of karst catchment carbon cycle monitoring and sink enhancement preferably comprises the preparation of the work programme, the survey of catchment structure, the monitoring of the carbon cycle process, the survey of artificial sink enhancement measures, the analysis of carbon sink intensity and flux and their evaluation by zone, the preparation of maps and the construction of the database. The flow is shown in Figure 1.

6 Content of monitoring and evaluation

6.1 The survey before monitoring covers relief, stratigraphy and structure, soil and vegetation, and hydrogeological features. For relief the survey covers elevation, slope, landform type and karst forms, so that their spatial distribution and association are established. For stratigraphy and structure it covers the rock association, the distribution of carbonate rock and of gypsum, sulphides and coal measures, so that the geochemical and structural type and position are established. For soil and vegetation it covers soil type, soil thickness, plant community type and composition, so that the ecological features are established. For hydrogeology it covers the hydrodynamic conditions and hydrochemical features of surface water and groundwater, so that the structure and boundary of the karst groundwater system are settled and the recharge, runoff and discharge conditions and the spatial layout analysed.

6.2 The monitoring of the conditions and processes of the carbon cycle covers six items. The conditions are air temperature, precipitation, potential evaporation, wind, solar radiation, net primary productivity of the vegetation and change of land use type, whose dynamic features are analysed. The soil carbon cycle process covers soil respiration, the carbon dioxide concentration of the air and the soil, and the dissolution amount of dissolution test tablets. The inorganic carbon cycle process of surface water and groundwater covers the migration of dissolved inorganic carbon in the underground karst aquifer medium and the change of carbon form, the flux and the source in caves, karst springs and underground rivers. The exchange and release of carbon dioxide at the surface water covers the release or absorption flux of surface rivers and its dynamic features. The conversion process of aquatic plants covers the conversion flux between organic and inorganic carbon in surface rivers and its dynamic features. The deposition process covers the spatial and temporal distribution of organic and inorganic carbon in lakes and reservoirs, the carbon deposition flux of the bottom mud, the deposition rate and its source.

6.3 The evaluation of artificial enhancement measures covers four items: the survey and evaluation of the increase of surface biological carbon sink from afforestation, grass planting and ecological restoration and of the resulting intensity of the underground karst carbon cycle and the carbon sink increment; the survey and evaluation of the rise of soil quality and the strengthening of the soil carbon cycle brought by soil improvement measures such as organic fertilizer, farmyard manure and biochar-based fertilizer, and of the strengthening of the karst carbon cycle below the soil and the carbon sink increment; the survey and evaluation of the intensity of the karst carbon cycle and the carbon sink increment brought by introducing allogenic water into the karst area; and the survey and evaluation of the artificial selection and breeding of aquatic photosynthetic organisms that raise the conversion efficiency from inorganic to organic carbon, increase the stability of the karst carbon sink and lower the release of carbon dioxide at the water-air interface.

7 Technical methods

7.1 The survey of catchment structure collects material on physical geography, meteorology and hydrology, regional geology, structural geology, hydrogeology and the ecological environment, and may collect material on the socio-economic situation, on the present use of water, soil, rock and mineral resources, on development plans and on other matters related to the carbon cycle. In important or unascertained ground such as the conduit medium of an underground river or the divide between underground rivers, geophysical exploration, drilling and tracer methods are preferably used so that the structure and boundary of the groundwater system are settled. The collected material is then analysed as a whole, the geochemical, geological and ecological factors that affect the carbon cycle of the catchment being summarized so that the key processes to monitor and the layout of the monitoring points become clear.

7.2 For the monitoring of the conditions of the carbon cycle, meteorological conditions are laid out with one point in each main climate type area and rainfall belt, monitored every 15 min or every 30 min; existing ecological and environmental meteorological station data are preferably collected, and where the accuracy falls short a fully automatic small meteorological station may be installed to obtain air temperature and precipitation and may also obtain potential evaporation, wind, solar radiation and atmospheric carbon dioxide concentration. Vegetation net primary productivity, land use change and river evaporation are covered over the whole area once a year, preferably obtained by remote sensing interpretation using multi-temporal data with little cloud, below 10 % cloud amount, and good interpretability, in the season of vigorous growth; net primary productivity may also be obtained by quadrat survey or computed from existing quadrat data, and in small catchments may be monitored by unmanned aircraft interpretation.

7.3 For the soil carbon cycle, the points for atmospheric and soil carbon dioxide concentration and for soil respiration are laid out in turn by stratigraphic lithology, vegetation type, land use mode and geomorphic position such as hilltop, hillside, depression and col, and are preferably distributed over the recharge, runoff and discharge zones of the groundwater system; monitoring is preferably carried out once in the dry season and once in the wet season each year, taking account of the dynamic features of hydrology and soil carbon dioxide. Carbon dioxide concentration is preferably measured by headspace bottle collection with high performance liquid chromatography, may be measured directly with a carbon dioxide meter, and automatic instruments may also monitor soil water content, temperature and carbon dioxide concentration every 15 min to 30 min; soil respiration may be measured by static chamber collection with high performance liquid chromatography or by a soil respiration meter, or existing published data may be collected. For dissolution test tablets the points follow the same rule, monitoring is once in each of the dry and wet seasons, and the method is to dig a soil profile and bury the tablets, digging only to the weathered layer where the soil is thin; one group of three tablets is buried in the air 100 cm above the ground, at the surface, 20 cm below the surface and 50 cm below the surface, each tablet being inserted into a small slot cut to its size; the dissolution amount is computed by weighing, and soil moisture, temperature and electrical conductivity are preferably measured on site. Soil physical and chemical properties follow the same point layout, are sampled once per catchment in the wet season, and the samples are taken when the tablets are buried, the laboratory preferably determining bulk density, pH, inorganic and organic carbon content and the stable isotope of organic carbon, and possibly nutrient elements, base ions, ammonium and nitrate nitrogen, other soil chemical components and loss on ignition.

7.4 For the inorganic carbon cycle of surface water and groundwater, routine water quality points are laid out along the groundwater flow direction by hydrochemical section, following the zoning of recharge, runoff and discharge, and sampling is preferably once in the dry season and once in the wet season each year. On site, pH, water temperature, electrical conductivity, dissolved oxygen, calcium ion, bicarbonate ion and total dissolved solids are preferably measured; a full analysis sample is taken at the first observation of a groundwater dynamic monitoring point and simplified analysis samples periodically thereafter. The simplified analysis covers the major cations and anions, and the full analysis adds a long list of further ions, trace elements and indices, ferrous iron, ammonium and nitrite being measured on site or fixed with a preservative. Dissolved organic carbon, total nitrogen, particulate organic carbon, the ratio of total organic carbon to total nitrogen and refractory organic carbon are also determined, the last following Annex D. Isotope methods are chosen according to the hydrogeological conditions and the question to be answered: nitrogen and oxygen isotopes of nitrate for the source of nitrogen, the stable isotopes of dissolved inorganic carbon and of particulate organic carbon for the source of inorganic and organic carbon, hydrogen and oxygen stable isotopes for the source of groundwater recharge, and sulphur stable isotopes for the environment of groundwater formation and the source of sulphur. The preservation, delivery and test methods of groundwater may follow GB/T 14848. The Galy method may be used to separate the weathering contribution of silicate from that of carbonate rock, and the mass balance method to separate the dissolution proportions of carbonic, sulphuric and nitric acid, so that the karst inorganic carbon sink flux formed by the dissolution of carbonate rock by carbonic acid is computed.

7.4 (continued) Dynamic monitoring of karst groundwater and surface water is deployed as a whole by groundwater system or surface catchment, so that the dynamic pattern of the working area is controlled; on regional rivers, discharge monitoring points or simple hydrological stations are deployed upstream, midstream and downstream, and monitoring is closer where surface and groundwater exchange frequently; springs are given points by type, aquifer group and discharge. The monitoring is preferably not shorter than one hydrological year. Water level is preferably monitored by automatic recorder; in areas of violent dynamic change, two or three rainstorm events are chosen and monitored at high frequency by automatic recorder, preferably not less than once every 30 min. Discharge observation of rivers, springs and flowing wells is preferably synchronous with groundwater level monitoring; simplified water quality analysis is carried out once or twice a month and every one to two hours during a rainstorm; and surface and groundwater temperature may be monitored once or twice a month, synchronously with level and discharge, so that the dynamic change of the karst carbon sink in the rainy season, the dry season and during rainfall is monitored.

7.5 to 7.7 For the exchange and release of carbon dioxide at the surface water, control sections are preferably chosen upstream, midstream and downstream at the outlet section of a surface or underground river; for lakes, including their groundwater recharge sources, the inflow area, the lake centre and the outflow area are preferably chosen; for reservoirs, the head, the middle and the tail. Sampling is preferably twice a year, in the dry and the wet season, where change is small, and four times a year where change is large, of which twice in the wet season, taking account of the dry, wet and normal seasons. On site, the discharge, water temperature, electrical conductivity, pH, calcium ion content, bicarbonate content, wind speed above the water surface and carbon dioxide concentration at the surface are preferably measured and a sample taken for simplified analysis; the release of carbon dioxide at the water-air interface may be computed from the concentration gradient between the air and the gas in the water body using the Fick law, and in small catchments static chamber collection with laboratory determination may also be used. The monitoring of the conversion process of aquatic plants uses the same point layout, is preferably carried out twice a year in the wet and dry seasons, and preferably sets quadrats for aquatic plant collection on typical rivers, describing the type, name and number of the plants qualitatively and computing the biomass; the daily variation of dissolved oxygen is monitored on site with a water quality meter, aquatic plant samples are tested for the ratio of total organic carbon to total nitrogen and for the stable carbon isotope of organic carbon, and water samples for temperature, electrical conductivity, pH, calcium ion, bicarbonate and the stable isotope of dissolved inorganic carbon; the flux and proportion of inorganic carbon converted to organic carbon by aquatic plants may be computed by the dissolved oxygen estimation method or the inorganic carbon isotope method. The monitoring of the deposition process uses the same point layout, is preferably sampled once per catchment in the normal season, and where the water is shallower than 20 m the organic carbon content, the stable carbon isotope of organic matter, total nitrogen, lead-210 and caesium-137 of the sediment are preferably determined; isotopes may be used to compute the carbon deposition flux and the deposition rate in rivers, lakes and reservoirs, the internal organic carbon deposition flux preferably being obtained by the binary model method.

7.8 The control experiments on artificial enhancement cover five items. From the land use interpreted by remote sensing, the treated area, leaf area index and net primary productivity of afforestation, grass planting and fruit planting under rocky desertification control are extracted and compared with historical data so that the vegetation recovery effect of the several measures is evaluated. Surface soil from 0 cm to 15 cm improved in different ways, by organic fertilizer, farmyard manure or biochar-based fertilizer, is sampled and standard dissolution tablets buried, the dissolution rates being compared so that the carbon sink benefit of the several improvement measures is evaluated, the soil indices being the same as for soil physical and chemical properties. Soils of treated and untreated karst areas with the same micro-habitat and soils under different improvement modes are chosen, and with a soil water collection device a carbonate rock dissolution experiment is carried out at the same time, the carbon sink being computed by the methods of comparable dissolution experiments and the hydrochemistry of the soil water serving as evidence, so that the enhancement effects of the several modes are compared; the soil water is measured for pH, calcium, magnesium and bicarbonate concentration, and where samples are numerous a simplified analysis may be made. For catchments with allogenic water input, the distribution and position of the clastic rock are surveyed and the discharge of the allogenic water monitored, water samples being taken along the course of pure allogenic water, mixed allogenic and karst water and pure karst water and analysed by the simplified analysis; the dynamic change of the water volume and the hydrogeochemical indices of the allogenic water and its dissolving capacity after mixing with karst water are monitored, and the sink increment of the allogenic water is computed by the hydrochemical runoff method at the same scale; a simulated irrigation experiment with allogenic water and karst water is carried out with standard dissolution tablets buried so that the dissolution rate is monitored. For catchments larger than 500 square kilometres, aquatic plant species of higher carbon fixation efficiency are selected and cultivated, gradient experiments in temperature, hydrodynamics and bicarbonate concentration are set up, and the change of hydrochemistry, dissolved oxygen and inorganic carbon isotopes is monitored, the indices being the same as for the aquatic plant conversion monitoring.

8 Accounting of the karst carbon sink and evaluation of its enhancement

8.1 The dissolution tablet method applies at every catchment scale, whereas the hydrochemical method differs with scale; the karst carbon sink intensity is the karst carbon sink flux divided by the area. Table 2 gives the accounting method by catchment area in three bands: not greater than 20 square kilometres, greater than 20 and not greater than 500 square kilometres, and greater than 500 square kilometres together with the band of 10 000 square kilometres and above. It fixes for each band the flux expression and the intensity expression, the flux being built from the dissolved inorganic carbon produced by the dissolution of carbonate rock by carbonic acid, to which the internal organic carbon sink flux is added in the second band, and the internal organic carbon deposition flux of lakes and reservoirs in the third. The note to the table gives the legend of the symbols and their units: the catchment carbon sink amount in tonnes of carbon dioxide per year; the catchment carbon sink intensity in tonnes of carbon dioxide per square kilometre per year; the karst groundwater runoff in litres per second; the catchment area in square kilometres; a dimensionless unit conversion coefficient; the dissolved inorganic carbon produced by the dissolution of carbonate rock by carbonic acid, in milligrams per litre; the inorganic carbon sink of the catchment produced by that dissolution, the internal organic carbon sink flux of the catchment and the internal organic carbon deposition flux of the lakes or reservoirs, all in tonnes of carbon dioxide per year; the corresponding three intensities in tonnes of carbon dioxide per square kilometre per year; the internal organic carbon concentration of the catchment in milligrams per litre; the area of the lakes and reservoirs of the catchment in square kilometres; and the dimensionless atomic weights of carbon dioxide and of carbon, 44 and 12. The formulae themselves are damaged in the extracted text and are not reproduced here.

8.2.1 For the evaluation of the controlling factors of the intensity of karst processes, a geographic information system serves as the platform for spatial and temporal data analysis, and spatial factor analysis and multivariate statistical analysis are used; taking the results of several years of karst carbon cycle monitoring over several catchments, the multi-year dynamic carbon stock and carbon flux of the karst catchment are computed. Through the identification of spatial relations by model, the effect on the carbon cycle of the relief, stratigraphic lithology and hydrodynamic conditions of the catchment together with the meteorological, vegetation, soil and human activity conditions is analysed, and the controlling factors of the intensity of karst processes in the several regions are evaluated.

8.2.2 For the evaluation of the increase of the karst carbon sink by artificial intervention, two methods are used. Artificial afforestation and grass planting, soil improvement, the introduction of allogenic water and other enhancement measures are evaluated by the comparison method, the difference in karst carbon sink intensity between treated and untreated ground and in the control experiments being compared with the carbonate rock dissolution tablet method so that the enhancement effect is assessed, the dissolution amount of the carbonate rock being computed by the standard tablet method. Artificial enhancement by the cultivation of aquatic plants is evaluated by direct computation, including the dissolved oxygen computation and the computation from inorganic carbon concentration and isotopes.

9 Construction of the database and preparation of results

9.1 A data platform is to be built and a monitoring database established so that the data are stored. The database is to have a detailed design, technical documents and metadata; the scope of the data collected may include monitoring attribute data, geospatial data, literature data and multimedia data; the relevant specifications on database construction are to be followed and the data standardized, loaded and updated; geospatial data are to use an internationally common geographic information system data format and platform; the extensible markup language data exchange format is to be supported; and the data are to be updated periodically or as needed. The data are of three classes: basic data products, covering regional geography such as latitude, longitude and elevation, geology such as stratigraphy, structure and lithology, meteorology such as rainfall, temperature and humidity, and hydrology such as discharge, electrical conductivity and pH; original data products, covering the basic information of the monitoring stations and the original monitoring data; and processed data products, covering statistical data, analytical data, map data, research reports, research papers and three-dimensional geological model data.

9.2 and 9.3 The original material, field test results, laboratory test, analysis and assay results and the intermediate integrated analysis results obtained by sorting, collection and monitoring are to be organized, and the several kinds of material compiled, the quantitative data being counted and correlated and special and integrated charts prepared. The integrated result database is built on a geographic information system platform; its geographic data are compiled from the database of the national geographic information centre; the geological professional layers are compiled chiefly from the southwest karst spatial database, from the results of surveys of groundwater and the ecological geological environment of karst rocky mountains and from the present monitoring material; layers such as soil distribution and vegetation type are compiled from existing maps and remote sensing interpretation results; and the external database construction contains all the monitoring and collected material that can be applied, with graphic libraries for the geological structure map, the geomorphological map, the groundwater resource distribution map, the groundwater hydrochemical map, the land use mode, the vegetation classification and the soil type, together with a database manual and a database construction report.

10.1 All maps are to be digitized as vector maps, the geographic base map using the data of the integrated spatial database of the national geographic information centre. The basic maps chiefly comprise the remote sensing image map, the geomorphological map, the map of the degree of karst development by zone, the map of groundwater system division, the map of groundwater resource distribution, the land use zoning map, the rocky desertification distribution map, the soil type map and the vegetation type map, which may be collected, merged or split according to the real situation. The result maps that are to be prepared are the map of the dynamic carbon stock distribution of the karst catchment and the table of the layout of karst carbon sink enhancement techniques; optional maps may be chosen according to the real situation of the monitored area, such as the map of carbonate rock dissolution rate by zone, the map of soil carbon dioxide concentration distribution and the map of vegetation biomass distribution.

10.1 (continued) The carbon stock distribution map is the result map that reflects the nature, source or sink, and the intensity of the carbon cycle of the karst catchment. Its basic content comprises the layers of geography and meteorological factors related to the karst carbon cycle, such as relief, drainage network and the contour lines of rainfall and temperature; the geological and hydrogeological conditions related to the cycle, such as lithological distribution, degree of karst development, hydrochemical features and groundwater runoff modulus; and the zoning of the carbon sink intensity of the karst catchment. From the results of the evaluation of karst carbon sink intensity and of its enhancement, together with the main influencing factors at catchment scale, the zoning of enhancement techniques and their layout table are proposed.

10.2 and 10.3 The result report is preferably to make full use of existing material and to reflect fully the results obtained by survey and testing; it is preferably concise, focused, well supported and clear in its conclusions, with complete drawings and attachments; and it is written according to the content laid down in Annex E. The review of the report is based on the project task statement, the design, the design review opinion, the field acceptance opinion and the relevant standards and requirements. After review, the report is to be revised carefully according to the review opinion, and the final report submitted to the approving body for examination and confirmation.