Boulder Municipal Airport (KBDU) has runways 8 / 26 and also runways 8G / 26G. What is the significance of the letter G following the runway numbers?
For runways with other than hard surfaces it is common to list the runways with a letter G for grass or in the case of a seaplane base, with W for water.
An example of this is runway 8G at Boulder (KBDU) and runway 35W at David Wayne Hooks (KDWH).
You can see this information in the Chart Supplement pages and diagrams for each airport.
The "G" suffix refers to a Glider runway.
The following verbiage is used in Advisory Circular: AC No: 150/5200-35 Page 15 RUNWAY IDENT
"The following suffixes can be used in conjunction with runway identification numbers even if the runway is not painted accordingly: S = STOL runway, G = glider runway, W = water sealane or waterway, and U = ultralight runway"
Interestingly runway 8G / 26G is actually a dual surface runway with part of it being paved and the remainder grass.
The following is the description of that runway: Found here AirNav: KBDU
Runway 8G/26G Dimensions: 4100 x 20 ft. / 1250 x 6 m
Surface: asphalt/turf, in fair condition
RWY 08G/26G 3,000X20 FT CHIP AND SEAL ON WEST END; EAST 1,100 FT IS GRASS.
Operational restrictions: RWY 08G/26G LNDG AREA UNDEFINED.
RUNWAY 8G RUNWAY 26G
Latitude: 40-02.378333N 40-02.378333N
Longitude: 105-14.001500W 105-13.122833W
Elevation: 5287.0 ft. 5274.0 ft.
Gradient: 0.3% UP
Traffic pattern: left right
Obstructions: 60 ft. trees, 200 ft. from runway, 126 ft. left of centerline, 3:1 slope to clear none